http://drupal.org/project/singlesignon there seems to be a conflict -> probably a session problem. Here is how to reproduce the 403 error:
- enable both modules
- log out
- log in
- upload 3 images -> first one will through a 403 error the others will work fine
- upload more images -> works fine, until logout and new login
- disable single sign-on module -> everything works fine

Since I hacked this module anyway... I would be totally fine with just a quick fix.