Cheers for that s33 - saved me a world of pain... Had the same issue, found your post, sorted in a few seconds.
I had 3 registrars in the public_html/whmcs/modules/registrars that were left over from years gone by.
For anyone else who has the same issue:
sort the public_html/whmcs/modules/registrars folder by date (oldest->newest).
Select any folders which are much older than the recent install. (I had 3 registrar modules from 2009 !!)
Archive them and download archive.
Delete those folders (the old ones).
Reload your configregistrars.php page and all should be working well.
(If not, you have a backup of any folders you removed, install one at a time, but the above should fix your issues)
Once again, thanks s33