Larian Studios made a unnecessary new bloatware ♥♥♥♥ launcher for the game and in the process broke the game on linux.

To fix it, open a terminal, cd to the installation directory:

Then run the following commands:

Thanks to Ordon for posting this information (originating from protondb[www.protondb.com])!
And ⎛⎝惡貫滿盈⎠⎞ for pointing out problems with the original solution!