The error is in the ubb template, it HATES the {* Script Version 7.5.7 *} at the top. I removed this and now the RSS feeds are working, so looks like this is a bug.
That should be filtered out by smarty... Are you sure there isn't a space " " after "{* Script Version 7.5.7 *}" ("{* Script Version 7.5.7 *} " versus "{* Script Version 7.5.7 *}")?
It's what I cleared out on the 9 sites I've updated today to fix it on those...