Hi Kaia:
You have already taken the steps that normally resolves the issue you are facing. So, from the top of my head, not sure what is the issue.
1. Try clearing the browser & server cache once -- for some users, clearing the cache after initial install resolved the problem.
2. If clearing the cache provides no result; could you check that there are no JavaScript errors (the attachment box is rendered using Ajax)?
3. If no JavaScript error, then could you enable MediaWiki debugging and check the log for database errors -- this may cause the Ajax call to fail
4. If don't see any error in the debug log, could you share the following information about your MediaWiki installation and I'll try to reproduce the error to resolve it:
1. Name of all the extensions installed
2. OS name & version
3. Skin name
Sincerely yours,
Aldrin