[blind-eddie]

All usergroups are having this issue, did you change where the avatars were uploading to? ie... database or file.
Did you add a modification that effected members image permissions or has its own permissions?

Shut off all plugins via FTP, includes/config. php

To do this, open the config.php in your favorite editor ,

Just below:

<?php

add below:

define('DISABLE_HOOKS', true);

Save.

Try to add custom avatar to see if you have the same error.