Adam,
Thankyou for the assistance. The table regeneration seems to have worked for several of the problem leads, except one. But to answer your question as to what was going on and where, through the stock "convert to customer" button, we did not receive the error. However, the leads were not actually converted when using that button. The lead still appeared as a lead and was not present in the customer list. Upon further inspection,I saw that several fields were updated such as conversion date. But "IsLeadStatusOnly" remained set as True.
The way the problem was detected was through a common procedure of ours involving the opportunities tab of the edit lead screen. Basically we used a custom Opportunity stage that mimicked the "closed won" stage. It was after setting this custom Opportunity Stage "Want List - No Purchase" that we would hit "Save" and be asked if we wanted to convert that lead to a customer. Previously, selecting Yes or Ok converted the customer. Last week, however, we began receiving the error message instead.
I'm unsure that it has any relation...but during this time I also noticed that several of the new dashboard screens (namely the Leads Dashboard, Activities Dashboard, and Cases Dashboard) mentioned in the release notes for Dec 30, do not appear. I have not tried looking through the tab link customization options yet. Again, I'm not sure if it has any relation but did wonder if there could have been a glitch during our update. The Opportunities Dashboard and Customers Dashboard appear as expected.
I'll do some further checking in regards to the conversion issue once I get up to speed with our end users. But in the meantime, MethodSupport is already enabled for our account. I'll email you our Company Account Name.
Update: The one lead that was remaining to be converted was indeed in QB. But the problem is now resolved after simply deleting the offending lead record. The converting issue appears resolved.
Thank you again,
James