I never got that error either , my issue was when changing a users avatar via admin cp i would have to go threw the paces 3 times or more before it would take , same as editing a forum or adding a new forum , it would take 15 mins before you could use it with out error messages from vbulletin , Vbulletin.com came to my admin cp with full access and returned with a report that they couldnt reproduce my issue , i immediately went to try to edit something and it took 15 mins before the changes were visible i did not report back to vb.com at that time
also we went threw and manually removed over 500 emails from user accounts that returned as undeliverable , to our surprise weeks later we went to look and all of them returned thats hours or wasted time and resources , which directly converts to $$$ LOST ,
again we manually processed the emails and only 200/500 took , it took us 3 more times before we was successful at removing the 500 ignoramus emails .
with 20-30 new members a day an no requirement to verify email accounts , every mass email turns up undeliverable mail , we let these build and build because our method of removal was hit and miss , the day i switched back to APC we manually processed about 100 emails all of them took first try