A new user on Premoderation posted the same message twice.
Via the Premod. function I tried to reject one message, and accept the other, and remove the user from Premoderation.

I ended up with a thread with the correct name, but no posts at all.
And the post I had tried to accept was still in Premoderation, but could no longer be accepted.

I had to make a dummy reply to the 'empty' thread, to then delete the thread.
And then reject the remaining post in Premoderation, and get the user to re-post the message.

Steve