Some files from the installed game could not be copied.

It says I probably don't care unless I have recently modified the game files - I haven't so I don't. BUT:

When I try to launch a co-op game it launches a custom game instead. ie: I can't choose the co-op map that I chose, just a custom map.

Error details:

java.util.concurrent.CompletionException: java.lang.RuntimeException: java.nio.file.AccessDeniedException: C:\ProgramData\FAForever\bin\ForgedAlliance.exe
at java.base/java.util.concurrent.CompletableFuture.encodeRelay(CompletableFuture.java:367)
at java.base/java.util.concurrent.CompletableFuture.completeRelay(CompletableFuture.java:376)
at java.base/java.util.concurrent.CompletableFuture$UniRelay.tryFire(CompletableFuture.java:1093)
at java.base/java.util.concurrent.CompletableFuture.postComplete(CompletableFuture.java:506)
at java.base/java.util.concurrent.CompletableFuture.completeExceptionally(CompletableFuture.java:2152)
at com.faforever.client.task.CompletableTask.lambda$new$1(CompletableTask.java:18)
at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:86)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:234)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:54)
at javafx.event.Event.fireEvent(Event.java:198)
at javafx.concurrent.EventHelper.fireEvent(EventHelper.java:219)
at javafx.concurrent.Task.fireEvent(Task.java:1358)
at javafx.concurrent.Task.setState(Task.java:709)
at javafx.concurrent.Task$TaskCallable.lambda$call$2(Task.java:1455)
at com.sun.javafx.application.PlatformImpl.lambda$runLater$10(PlatformImpl.java:428)
at java.base/java.security.AccessController.doPrivileged(AccessController.java:391)
at com.sun.javafx.application.PlatformImpl.lambda$runLater$11(PlatformImpl.java:427)
at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:96)
at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
at com.sun.glass.ui.win.WinApplication.lambda$runLoop$3(WinApplication.java:174)
at java.base/java.lang.Thread.run(Thread.java:832)
Caused by: java.lang.RuntimeException: java.nio.file.AccessDeniedException: C:\ProgramData\FAForever\bin\ForgedAlliance.exe
at com.faforever.client.patch.SimpleHttpFeaturedModUpdaterTask.lambda$call$0(SimpleHttpFeaturedModUpdaterTask.java:81)
at java.base/java.util.ArrayList.forEach(ArrayList.java:1511)
at com.faforever.client.patch.SimpleHttpFeaturedModUpdaterTask.call(SimpleHttpFeaturedModUpdaterTask.java:65)
at com.faforever.client.patch.SimpleHttpFeaturedModUpdaterTask.call(SimpleHttpFeaturedModUpdaterTask.java:24)
at javafx.concurrent.Task$TaskCallable.call(Task.java:1425)
at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1130)
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:630)
... 1 more
Caused by: java.nio.file.AccessDeniedException: C:\ProgramData\FAForever\bin\ForgedAlliance.exe
at java.base/sun.nio.fs.WindowsException.translateToIOException(WindowsException.java:89)
at java.base/sun.nio.fs.WindowsException.rethrowAsIOException(WindowsException.java:103)
at java.base/sun.nio.fs.WindowsException.rethrowAsIOException(WindowsException.java:108)
at java.base/sun.nio.fs.WindowsFileCopy.copy(WindowsFileCopy.java:164)
at java.base/sun.nio.fs.WindowsFileSystemProvider.copy(WindowsFileSystemProvider.java:283)
at java.base/java.nio.file.Files.copy(Files.java:1299)
at com.faforever.client.io.FeaturedModFileCacheService.copyFeaturedModFileFromCache(FeaturedModFileCacheService.java:64)
at com.faforever.client.patch.SimpleHttpFeaturedModUpdaterTask.lambda$call$0(SimpleHttpFeaturedModUpdaterTask.java:74)
... 8 more

Download the client again and update it.
Worked for me.