I fiddled around with this a little and I think it may have to do with my "special" setting of the brightness value, which i set to "r_gamma 3".
Now everytime my screen was too dark after I maximized and I executed "/set r_gamma 3" again, nothing happened.
BUT! If I set it to "r_gamma 2" and then to "r_gamma 3" it magically works.
So I made a toggle script for the gamma settings:
bind F10 "vstr gamma_up"
bind F11 "vstr gamma_down"
set gamma_up "vstr gamma_20
set gamma_down "vstr gamma_30
set gamma_20 "r_gamma 2.0; echo Gamma: 2.0; set gamma_up vstr gamma_25; set gamma_down vstr gamma_30"
set gamma_25 "r_gamma 2.5; echo Gamma: 2.5; set gamma_up vstr gamma_30; set gamma_down vstr gamma_20"
set gamma_30 "r_gamma 3.0; echo Gamma: 3.0; set gamma_up vstr gamma_20; set gamma_down vstr gamma_25"
Case closed for me, but feel free to comment if you have any idea WHY that is.