I have ran up against a wall trying to get the forum back to normal. YaBB Support is adamant it is a host, or other external issue, and I tend to agree, since this forum has run without error for years except for this and the earlier login issue that mysteriously fixed itself.
I can only suspect it is the host so I am looking to maybe move the forum to a different host. I may be able to get an "unlimited" hosting package for $7.50/month...