Error trying to post in Kunena

We are using Kunena and GroupJive. Occasionally when trying to submit a post in Kunena the user gets this error:

message: You cannot leave this field empty

When this happens, the text box on the Kunena screen is highlighted. The text which the user tried to enter is lost and cannot be retrieved.

I wondered if the problem might be due to a problem with the character set, or with some character sequence being misinterpreted by Kunena as a code. Sometimes the user may have been trying to paste something in from another webpage. However, it has also happened when all the text has been typed directly into the Kunena page.

The forum is on a restricted part of our site, so unfortunately I am unable to demonstrate.

Any ideas? Our set up:

Joomla 1.5.23
CB 1.7
CB Subs 1.2.2
CB GroupJive 2.4
Kunena 1.7

Browser: Firefox.

GJ only affects access permissions and has no barring on the post it self. To confirm conflict isn't present please disable GJs forum integration and re-test. I recommend posting to Kunena forums for further support. I've let them know of this thread however as it could be a bug from either side (will better know once confirmed by disabling forum integration).