When at [[m:Special:UserRights]] trying to apply xwiki rights (as a steward) and it fails, generating a FATAL ERROR screen
PHP fatal error in /usr/local/apache/common-local/php-1.23wmf13/includes/specials/SpecialUserrights.php line 155:
Call to undefined method UserRightsProxy::clearInstanceCache()
Stewards are therefore unable to change any rights crosswiki.
I was unsuccessful to apply rights changes to idwikibooks (different rights tried), mediawiki, afwiki. This is wmf12 and wmf13 renditions, so it doesn't seem to be at the receiving end.
I can successfully change local rights at meta, so it does not seem to be internal to a wiki.