Every newsgroup message (as well as emails) have a From header to say who the message came from. OE6 has seemed to have forgotten who it is sending the message.
Right-click your newsgroup account (not the newsgroups, the whole thing in the Folders view), and select Properties. In the box that appears, fill in the Name and Email address boxes. The name doesn't have to be a real one, just a username or display name for you. Mine is QuickHare, for example. The Email address box can also be faked, but remember anyone sending you an email from the group will use this address. I mask mine by adding spurious characters that obviously shouldn't be there, so a human can still read it, but it fools a computer.
Anyway, I digress.
Just pop your details in these boxes and click OK, and hopefully it should work.