It seems to be a bug. Under what circumstances it occurs - some
I posted it on the Yabb support forums & got no replies (....) It may have occurred due to the transfer from our old server.
The workaround is to 'PM' the person rather than 'reply' ... Not clean & many will encounter the problem.
Sooner or later I'm intending to transfer these forums to a PHP based alternative. Simply because the Yabb support is so weak. Problem with this 'free' forum (yabb is needlessly & stupidly free) is that support is just not there. I wish they'd charge & be there .....
-- So - soon time to move on ..