I had the same issue now after my GOG Galaxy 2.0 Update on Windows 10 machine. Starting in Safe Mode once solved the problem.

I've got the same problem. Delete Galaxy Key and restart the computer help :). Thanks for this information :)

when you use {any} Windows build there are at least 2 accounts
Your account which may or may not be admin +
system account i.e, the one Microsoft uses to backdoor into your computer to run updates and such... as you say you have 8.1 Microsoft will be downloading | scanning for the W10 upgrade

This issue popped up on my computer this morning at start up.

Before deleting anything try this.

What I did to resolve this issue was simply running GOG client as an administrator once.

It started up with zero problems.

I then shut down the client and restarted the computer.

Clicked on the client icon and it started up normally.

It appears some kind of update from someone has reset the client. I had to enter a code to verify my account just to post this.

This is a Windows problem, not a Galaxy one.
EDIT: I just replied to a 4 year old comment.... *sigh*

Go to C:\ProgramData\GOG.com\Galaxy\lock-files and delete the GalaxyClient.exe.lock file and try launching Galaxy again

Thanks JTQuincy, this worked and is very simple.

Me too :-)
this occurs with galaxy only, and deleting a lock file (as suggested in the thread) works, so this is a galaxy problem...

THX very much

This worked for me, too. Thank you!

Great solution, everything is back to normal. Thanks for your help JTQuincy!

It's definitely a galaxy problem. This is their own custom lock file and the app isn't even robust enough to handle the scenario that the program shuts down abnormally, failing to clean up the lock file (what happened to me), and the NEED for a lock file is 100% due to the way they designed this application and what it does, and has nothing to do with Windows.

I remember having this problem, one of the way sI think you can get around it is with named shared memory. You can check when you start up if you can grab access to the memory, if you can then you are the only instance running. If you can't then there is another instance running and you need to shut down.

Thanks JT. I worked for me too.

I hope GOG is looking into this problem (the main reason I replied is so that they can see this error is occurring for a lot of people).