Garry's mod not saving settings

alright, here’s my problem:

whenever I boot up I have to change the screen resolution, graphic levels…manually, always because in the console it says something along the lines of “exec: could not write to userconfig.cfg” twice, once when changing the settings and once when clicking ok.

You have to quit the game normally for the settings to be saved.

It writes settings changes to the file on exit, so it won’t work if you crash.

I always exit normally (no crashing or whatever, after my play session I press esc and click "quit game), but what should I do?