@gjc
Check and see if
vsa_advreg_invite is created in your DB.
If it did not, I would do a complete uninstall an then a re-install of the mod.
If you still are having issues, then there must be a mod conflict although you say you have check them and found nothing.
I have been running this mod for a long time and currently running it on 4.1.1 and I have never had issues at all.