Since the last 2 updates I experience a strange behaviour. It happens at random and I can't understand exactly when.
Sometimes when I start GoG Galaxy, none of my keyboard keys are working except the arrow keys. Unpluging and pluging the keyboard doesn't help - only windows restart.

OS: Windows 8.1
Keyboard: CM Rapid i

This has been reported a few times already, but I'm not sure there is a solution yet.

http://www.gog.com/forum/general/keyboard_issues_everytime_gog_galaxy_updates

Thank god, i'm not the only one. Found this thread with google, had this bug 3 times. 1st time while playing The Witcher 3, 2nd time while updating Pillars of Eternity, 3rd time directly after starting The Witcher 3. What it does:

Keyboard and mouse buttons refuse to do what they normally do. Instead the ESC-key opens the windows start menu (i'm using Windows 7 64 bit btw.), letter keys don't work, mouse buttons do weird things like not opening links in the browser etc.

I already suspected it might be some kind of virus because i did not think of GOG-Galaxy as cause of the problem. Did several scans of my system, but everything looks clean. After this happened now three times, every time while doing something GOG-Galaxy related, i get the suspicion it might be a weird bug in GOG-Galaxy.

*edit*

Oh, and btw, after i closed GOG-Galaxy this time, everything went normal again, while i had to reboot my computer the first two times.

You can report all bugs in this section so the devs can work on the issues :)

http://mantis.gog.com

I added an issue in the mantis now. I hope this can be fixed soon, because it is getting really annoying to restart the OS each time.

Another person had a similar problem to yours, but was able to solve it by installing the preview update.