star - Thanks for the information, given this is surrounding the datastore you can opt of using it within vBulletin (in the config.php line, comment out: $config['Datastore']['class'] = 'vB_Datastore_Memcached'

. If the bug doesn't appear then, it's the vBulletin Datastore.
Alternatively, you can disable Datastore Cacher on vB Optimise - however this only forces custom datastore items to use the correct datastore method (as vb doesn't handle them correctly).
Let me know if I can be of any further help, Memcache will surpass Filecache immensly (Memcache uses RAM, Filecache uses your HDD) so it'd be great if you got it working without problems