I had this problem too and I tracked it down to the serialize / unserialize code in rss.php. For some feeds, it appears that the serialized objects are corrupted during the mysql roundtrip (not entirely sure why?) and the quick solution is to base64 encode and decode the serialized objects as they pass in and out of mysql via updateOption and getOption. My fix is available via github if anyone is interested. It also fixes up some of the broken attempts to write out debug messages in the RSSCache.
