It seemed as Feed Validator didn't like me. When I checked it said it can't validate my RSS feed. However, loading my feed within a web browser looked ok. Therefore, I wrote an email to Mark and Sam this morning. I've got following reply:
Yeah, that's a known bug. It's not your fault at all. It's a server configuration issue on the new server, and Sam and I are actively working on a fix. (It affects his feed too.)
Thanks for the report. We'll announce at http://feedvalidator.org/news/ when we have a fix.
Thanks in advance, Mark.
PS: BTW, I cannot find a RSS feed for the Feed Validator News