XF 1.5 Users cannot register on my website

Then error given appears like it may be tied directly back to it. You will need to contact the author of that add-on, but in the mean time you can probably disable it and registrations should work.

EDIT:
If it worked after deactivating and reactivating it then odds are the add-on had a small hiccup during initial install. I'd still let the author know about the issue.
 
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