Try manually deleting all files in the core/cache directory again.
If that doesn't do it and you haven't changed anything yourself (e.g, .htaccess, FURLs, config.inc.php, config.core.php), I don't know what to suggest.
One possible cause is that in the upgrade, one or more new files did not overwrite the existing ones, either because of the method used to put them in place, or because of file/directory permissions or ownership.
You asked if I had seen this. Personally, I've never had a problem that clearing the cache and cookies didn't fix. Maybe because I've always hosted on
MODX-friendly hosts.