Alright, I had some sort of weird troubles uploading pictures in the past, which made me re-install the gallery. I moved the "albums" section into the new gallery folder, deleted the already existing normal and thumb files that belonged to the old gallery and added the pictures to the new gallery. Well, the whole gallery is set up again, but another error occurred when I tried to upload new images today.
I used the search to see if anyone already had the same problem and it's pretty much to same as here:
http://forum.coppermine-gallery.net/index.php/topic,33167.msg154705.html#msg154705 but since I am using the 1.5x version I created a new topic.
Anyway, when I try to add new pictures to an album (doesn't matter whether it's per batch or the regular uploader) the image is being added, but there doesn't seem to be an orig_ normal_ or thumb_ file. When I take a look at the folder via FTP it seems like the files have been created, but they're just blank files. I remember that it worked a while ago when I tried it and I actually don't remember changing any settings that might have caused the problem.
If anyone wants to try it, that's the gallery:
http://marina-diamandis.org/gallery/and I've created a test-account:
Username: tester
Password: testuser
It'd be really great if anyone could help me, I really don't want to set up the gallery for the third time, it would just takes ages again :/