Mark,
I'm currently using DNN 5.4.4 when I checked the users table I noticed that CreatedByUserID, CreatedOnDate, LastModifiedByUserID and LastModifiedOnDate are fields listed last on the table. I would normally included these fields first; therefore I suspect these were added during a DNN update.
I would check your users table structure. If these fields are missing consider upgrading your DNN installation or adding these fields manually.
CreatedByUserID int allow nulls
CreatedOnDate datetime allow nulls
LastModifiedByUserID int allow nulls
LastModifiedOnDate datetime allow nulls
I'm not sure what version of DNN started using these fields, something you would have to ask DNN Users support group or Data Springs.
Kris