Cannot upload anything in the file manager

[m1k3y]

I have a strange problem with GS 3.0,the site now worked for nearly a year and now i cannot upload anything.
If i choose a file to upload it shows the progressbar and says "Complete" but i don't see the file in the list...
Also i cannot generate new Folders...
Whats wrong here?
I'll try to upgrade to GS 3.1 and report here if its working but i'd like to know what went wrong... Ideas?