Adding sub-section results in "Unable to process your request" message

Adding sub-section results in "Unable to process your request" message

I'm trying to add a Security sub-section to my KB, but it results in a "Unable to process your request" message popping up.

If I edit an article, and try to create the sub-section that way, it reports that "This name is taken"

I have a feeling I created it quite some time ago and deleted it, but can't find it in recycle bin to recover.

Any suggestions?

Thanks,
Angus.