Problem posting a new topic to a Group

I've installed Dolphin 6.1.0 and also upgraded to 6.1.1 and still get the same problem.

As an ordinary user I've created a new Group but can't seem to Post a Topic to the group.
When I click on the Post topic button I get an 500 Internal Server Error.

As the URL looks like this

<domain.name>/groups/orca/forum/Website-0.htm#action=goto&new_topic=Website

I'm wondering if there could be another problem with the .htaccess file in the /groups/orca folder?

I've tried switching of the search engine friendly URL's but the URL on this button remains the same (the URL's on the other buttons on the grp.php page have changed).

An ideas folks?