Unable to write to /config.php

I'm updating the gallery from version 1 to 131. So, I unzip and overwrite all the files in the current gallery directory. I'm Using Apache on Unix. I run the configure and then go to the setup wizard. Everything is fine, no error, until I try to save the configiration when I get the above mentioned error.
I could tell, by looking at the erroring code, that the global variable $GALLERY_DIR isn't set. This would be ok, because I can set that in the code. But then, when I go to the secure mode and view my galleries, everything is fine and working except some global variables - like the Name of the Gallery and album in the Header!
Did somebody have simmilar problem? Does anyone know, what coud case this?
I actually installed this version also on NT with Apache and did the update, and there everything went well.

HELLPPPP!!!!! :roll: