The problem is gone now after a regular reinstallation of the client.
Thank you all for the replies.

@zars
Have checked all of that. Didn't help.
Can it be a problem with client v2023.11.1?

Didn't see that advice, trying to check it.

@magge said in Uh Oh Game Could Not Be Joined:

Reason
Program is blocked or has wrong user rights
Make sure security software is not blocking the client
Make sure your windows account has rights to run the software
Remove "C:\ProgramData\FAForever" folder; and reinstall FAF
For testing purpose, run the client with admin rights.

Getting a constant error when trying to join or create my own game (not modded).
Does anybody have an idea how to fix this?

Detatils of the error:
"Game could not be started. See below:
Client Version: 2023.11.1
java.lang.NullPointerException: Cannot invoke "java.lang.Process.onExit()" because "process" is null
at com.faforever.client.game.GameService.spawnTerminationListener(GameService.java:711)
at com.faforever.client.game.GameService.lambda$startGame$58(GameService.java:697)
at java.base/java.util.concurrent.CompletableFuture$UniCompose.tryFire(CompletableFuture.java:1150)
at java.base/java.util.concurrent.CompletableFuture.postComplete(CompletableFuture.java:510)
at java.base/java.util.concurrent.CompletableFuture.completeExceptionally(CompletableFuture.java:2162)
at reactor.core.publisher.MonoToCompletableFuture.onError(MonoToCompletableFuture.java:77)
at reactor.core.publisher.FluxOnAssembly$OnAssemblySubscriber.onError(FluxOnAssembly.java:544)
at reactor.core.publisher.MonoCollectList$MonoCollectListSubscriber.onError(MonoCollectList.java:108)
at reactor.core.publisher.FluxOnAssembly$OnAssemblySubscriber.onError(FluxOnAssembly.java:544)
at reactor.core.publisher.Operators$MultiSubscriptionSubscriber.onError(Operators.java:2210)
at reactor.core.publisher.FluxOnAssembly$OnAssemblySubscriber.onError(FluxOnAssembly.java:544)
at reactor.core.publisher.Operators.error(Operators.java:198)
at reactor.core.publisher.FluxError.subscribe(FluxError.java:43)
at reactor.core.publisher.Flux.subscribe(Flux.java:8773)
at reactor.core.publisher.FluxSwitchIfEmpty$SwitchIfEmptySubscriber.onComplete(FluxSwitchIfEmpty.java:82)
at reactor.core.publisher.FluxOnAssembly$OnAssemblySubscriber.onComplete(FluxOnAssembly.java:549)
at reactor.core.publisher.FluxSwitchIfEmpty$SwitchIfEmptySubscriber.onComplete(FluxSwitchIfEmpty.java:85)
at reactor.core.publisher.FluxOnAssembly$OnAssemblySubscriber.onComplete(FluxOnAssembly.java:549)
at reactor.core.publisher.FluxFlattenIterable$FlattenIterableSubscriber.drainAsync(FluxFlattenIterable.java:371)
at reactor.core.publisher.FluxFlattenIterable$FlattenIterableSubscriber.drain(FluxFlattenIterable.java:724)
at reactor.core.publisher.FluxFlattenIterable$FlattenIterableSubscriber.onComplete(FluxFlattenIterable.java:273)
at reactor.core.publisher.FluxOnAssembly$OnAssemblySubscriber.onComplete(FluxOnAssembly.java:549)
at reactor.core.publisher.FluxPeekFuseable$PeekFuseableSubscriber.onComplete(FluxPeekFuseable.java:277)
at reactor.core.publisher.FluxOnAssembly$OnAssemblySubscriber.onComplete(FluxOnAssembly.java:549)
at reactor.core.publisher.MonoPeekTerminal$MonoTerminalPeekSubscriber.onComplete(MonoPeekTerminal.java:299)
at reactor.core.publisher.FluxOnAssembly$OnAssemblySubscriber.onComplete(FluxOnAssembly.java:549)
at reactor.core.publisher.SerializedSubscriber.onComplete(SerializedSubscriber.java:146)
at reactor.core.publisher.FluxRetryWhen$RetryWhenMainSubscriber.onComplete(FluxRetryWhen.java:200)
at reactor.core.publisher.FluxOnAssembly$OnAssemblySubscriber.onComplete(FluxOnAssembly.java:549)
at reactor.core.publisher.MonoFlatMap$FlatMapMain.secondComplete(MonoFlatMap.java:246)
at reactor.core.publisher.MonoFlatMap$FlatMapInner.onNext(MonoFlatMap.java:305)
at reactor.core.publisher.FluxOnAssembly$OnAssemblySubscriber.onNext(FluxOnAssembly.java:539)
at reactor.core.publisher.FluxOnAssembly$OnAssemblySubscriber.onNext(FluxOnAssembly.java:539)
at reactor.core.publisher.FluxOnErrorResume$ResumeSubscriber.onNext(FluxOnErrorResume.java:79)
at reactor.core.publisher.FluxOnAssembly$OnAssemblySubscriber.onNext(FluxOnAssembly.java:539)
at reactor.core.publisher.FluxOnAssembly$OnAssemblySubscriber.onNext(FluxOnAssembly.java:539)
at reactor.core.publisher.FluxOnAssembly$OnAssemblySubscriber.onNext(FluxOnAssembly.java:539)
at reactor.core.publisher.FluxOnAssembly$OnAssemblySubscriber.onNext(FluxOnAssembly.java:539)
at reactor.core.publisher.FluxOnAssembly$OnAssemblySubscriber.onNext(FluxOnAssembly.java:539)
at reactor.core.publisher.FluxOnAssembly$OnAssemblySubscriber.onNext(FluxOnAssembly.java:539)
at reactor.core.publisher.FluxOnAssembly$OnAssemblySubscriber.onNext(FluxOnAssembly.java:539)
at reactor.core.publisher.FluxOnAssembly$OnAssemblySubscriber.onNext(FluxOnAssembly.java:539)
at reactor.core.publisher.FluxOnAssembly$OnAssemblySubscriber.onNext(FluxOnAssembly.java:539)
at reactor.core.publisher.MonoFlatMap$FlatMapMain.onNext(MonoFlatMap.java:158)
at reactor.core.publisher.FluxOnAssembly$OnAssemblySubscriber.onNext(FluxOnAssembly.java:539)
at reactor.core.publisher.FluxContextWrite$ContextWriteSubscriber.onNext(FluxContextWrite.java:107)
at reactor.core.publisher.FluxOnAssembly$OnAssemblySubscriber.onNext(FluxOnAssembly.java:539)
at reactor.core.publisher.FluxMapFuseable$MapFuseableConditionalSubscriber.onNext(FluxMapFuseable.java:299)
at reactor.core.publisher.FluxOnAssembly$OnAssemblySubscriber.onNext(FluxOnAssembly.java:539)
at reactor.core.publisher.FluxFilterFuseable$FilterFuseableConditionalSubscriber.onNext(FluxFilterFuseable.java:337)
at reactor.core.publisher.FluxOnAssembly$OnAssemblySubscriber.onNext(FluxOnAssembly.java:539)
at reactor.core.publisher.Operators$BaseFluxToMonoOperator.completePossiblyEmpty(Operators.java:2071)
at reactor.core.publisher.MonoCollect$CollectSubscriber.onComplete(MonoCollect.java:145)
at reactor.core.publisher.FluxOnAssembly$OnAssemblySubscriber.onComplete(FluxOnAssembly.java:549)
at reactor.core.publisher.FluxOnAssembly$OnAssemblySubscriber.onComplete(FluxOnAssembly.java:549)
at reactor.core.publisher.FluxMapFuseable$MapFuseableSubscriber.onComplete(FluxMapFuseable.java:152)
at reactor.core.publisher.FluxPeekFuseable$PeekFuseableSubscriber.onComplete(FluxPeekFuseable.java:277)
at reactor.core.publisher.FluxOnAssembly$OnAssemblySubscriber.onComplete(FluxOnAssembly.java:549)
at reactor.core.publisher.FluxOnAssembly$OnAssemblySubscriber.onComplete(FluxOnAssembly.java:549)
at reactor.core.publisher.FluxMapFuseable$MapFuseableSubscriber.onComplete(FluxMapFuseable.java:152)
at reactor.core.publisher.FluxOnAssembly$OnAssemblySubscriber.onComplete(FluxOnAssembly.java:549)
at reactor.core.publisher.FluxOnAssembly$OnAssemblySubscriber.onComplete(FluxOnAssembly.java:549)
at reactor.netty.channel.FluxReceive.onInboundComplete(FluxReceive.java:413)
at reactor.netty.channel.ChannelOperations.onInboundComplete(ChannelOperations.java:444)
at reactor.netty.channel.ChannelOperations.terminate(ChannelOperations.java:498)
at reactor.netty.http.client.HttpClientOperations.onInboundNext(HttpClientOperations.java:753)
at reactor.netty.channel.ChannelOperationsHandler.channelRead(ChannelOperationsHandler.java:114)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:444)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:420)
at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:412)
at io.netty.channel.CombinedChannelDuplexHandler$DelegatingChannelHandlerContext.fireChannelRead(CombinedChannelDuplexHandler.java:436)
at io.netty.handler.codec.ByteToMessageDecoder.fireChannelRead(ByteToMessageDecoder.java:346)
at io.netty.handler.codec.ByteToMessageDecoder.channelRead(ByteToMessageDecoder.java:318)
at io.netty.channel.CombinedChannelDuplexHandler.channelRead(CombinedChannelDuplexHandler.java:251)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:442)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:420)
at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:412)
at io.netty.handler.ssl.SslHandler.unwrap(SslHandler.java:1471)
at io.netty.handler.ssl.SslHandler.decodeJdkCompatible(SslHandler.java:1334)
at io.netty.handler.ssl.SslHandler.decode(SslHandler.java:1383)
at io.netty.handler.codec.ByteToMessageDecoder.decodeRemovalReentryProtection(ByteToMessageDecoder.java:529)
at io.netty.handler.codec.ByteToMessageDecoder.callDecode(ByteToMessageDecoder.java:468)
at io.netty.handler.codec.ByteToMessageDecoder.channelRead(ByteToMessageDecoder.java:290)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:444)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:420)
at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:412)
at io.netty.channel.DefaultChannelPipeline$HeadContext.channelRead(DefaultChannelPipeline.java:1410)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:440)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:420)
at io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:919)
at io.netty.channel.nio.AbstractNioByteChannel$NioByteUnsafe.read(AbstractNioByteChannel.java:166)
at io.netty.channel.nio.NioEventLoop.processSelectedKey(NioEventLoop.java:788)
at io.netty.channel.nio.NioEventLoop.processSelectedKeysOptimized(NioEventLoop.java:724)
at io.netty.channel.nio.NioEventLoop.processSelectedKeys(NioEventLoop.java:650)
at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:562)
at io.netty.util.concurrent.SingleThreadEventExecutor$4.run(SingleThreadEventExecutor.java:997)
at io.netty.util.internal.ThreadExecutorMap$2.run(ThreadExecutorMap.java:74)
at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30)
at java.base/java.lang.Thread.run(Thread.java:833)"