Well, I didn't noticed it untill now.
When i want to download the zip file from an e-mail attachment, I get this beautiful error:

Unable to Download Attachment
Internal error:

Error Details:
Error code 10000:InsertCacheObject: Could not find '61111616' bytes for object insert. (MojErr -199 -- src/commands/FetchPartCommand.cpp

As far as I understand, the cache is too small. But the zipfile is only 15 MB big. Cleaned the cache, problem didn't go away. Anyone a solution? :-)