Well, after changing the RAM value further and later configuring the server I can safely say it is working now. Thanks for helping me out, I really had quite the chance to learn more about linux trying everything out.

Key - Gas Station highlights the intact doors, but not the doorknobs from broken doors on the ground. I think it used to do this. Otherwise there is no way to know if you have the right key for the doorknobs for a house with all broken doors.  

Version; 42.19 Mode; Singleplayer Server Settings: none Mods: none Save: new save made on 42.19 Steps of reproduction: Join new singplayer world. Do anything that makes you wet, or just wait for rain of any sort. Stay under the rain, character wont experience any wetness.

I did set it to 6 GB and sadly to no avail, the error log is still the same.   

Took me a while to catch this on video, but here. Notice a big drop when zombie falls on the ground and then shuffles to face towards me, ingame FPS counter doesn't show it but steam FPS counter on top left does. Project Zomboid FPS drop.mp4