I found many threads about this problem, most unresolved, dating back to early last year. It started (for me) when the user activation emails stopped going out for no apparent reason. I tried all the suggestions I found in the forums to no avail. There was no debug information, and no visible reason for the problem.

Then, after I re-uploaded all the core files and updated the system and member modules, everyone who tried to register got the error: "Could not register new user" - Still no debug information and I was pretty frustrated. I thought about re-building the whole site, but I am glad I didn't.

Finally - I went into phpMyAdmin, and compared the structure of that user database with one that works. I found a simple problem, fixed it, and now the activations and registrations work fine.

The fix:

1) Opened XOOPS database in phpMyAdmin.
2) Browsed the table xoops_users.
3) Clicked on the "structure" tab at the top.
4) Changed the default value for "level" from '1' to '0'

Once I saved that everything went back to normal and now everything works fine again. I don't know HOW that value got changed in the first place, as it all worked fine a week ago. Regardless, that fixed it for me. Maybe someone else is dealing with the same problem, and this will help.

thanks for the heads up - maybe it will come in useful for someone in future :)

i haven't come across that exact problem before altho i have had issues with "could not register new user" it wasn't your solution in my case(s) what was the problem..

but it's 1 more thing to try if others have the same :)