Problem with unpacking files in Mountain Lion (Error 2)

When I try to pack or unpack files there only come up errors since I updated to ML.

"Error 2 - no file or folder existing". Seems to be a sandbox issue, I tried "The Unarchiever.app" and there it said the OS-sandbox doesn't allow this action. I'm getting kind of annoyed by all the ML-bugs, more problems in the last 7 days since updating than in the 2 years before using Snow Leopard and Lion...

Can anyone help me with this one?

Thanks!