Hi Jakub,
Make sure the wp-content/uploads and wp-content/cache directories exist and are writeable, that should very likely resolve the issue.
This is a known problem, there is a missing line of code to supress any directory creation errors, it will be patched in the next release. I can patch it for your locally via FTP access if you want to. If you set the permissions for the upperr mentioned folders, then it should go away either way.