I have the same problem and can not create categories, this is a fresh Groupjive install on a recently updated CB, CB Subs and Joomla from 1.5.15 to 1.5.22.
PHP Version: 5.1.6
Joomla! 1.5.22
CB 1.3.1
CB Subs 1.1.2
Groupjive 1.8
I enabled legacy and it's still the same, cleared the cache also, still the same.
I don't have any access to logs on this host.
Any ideas?
regards,
Andrew