If you're using Kunena v2.0, right now our profile plugin hasn't been updated for that version of Kunena. It only works with Kunena 1.x. We'll be releasing an updated v2.0 compatible plugin very soon. Until then, you should disable that plugin if so.
If not, check to make sure that the following directories are writeable:
* Your Joomla tmp directory (can check in Site Info -> Directory Permissions)
* /media/kunena/avatars/users (for Kunena 1.6. If using 1.5, let us know, that's a different directory). You'll need to check this permission on the server itself through either FTP or command line (SSH).
Hope that helps understand the problem more!
Thanks,
Alex