It appears there is either another instance already running or something else is using port 8080 which the client needs
Why would the client need port 8080 not like it is a web server???? Maybe it is miss-configured to run a web server??? Introducing new libs can have this effect.
There was an added dependency to run open discord from the client using a web socket. However this led to the client believing it was also a webserver due to auto configuration options. It will be fixed in the next release.
If you have this issue and cannot stop the process on port 8080 you can use the next beta release which should not need port 8080 found here https://github.com/FAForever/downlords-faf-client/releases/tag/v1.4.3-RC2