OK, first of all, I've looked to (maybe) every post regarding this issue. Tried everything, but nothing worked. I'm running version 2.0.7.3 with little chance to upgrade (yet). I got this issue after migrating to a new server, so I assumed I screwed something up in the process, as most of the posts here led me to think. Well, I didn't see anything like this in other posts, so I'm posting it now: version 2.0.7.3 DOES NOT WORK with PHP 5. Several pages use get_class() function and compare the results with a hardcoded, all lower case, string. The bad thing is that classes are named like "XoopsUser" instead of "xoopsuser". This behavior was introduced in PHP5. It turns out that the query never makes it to the database because it fails in the first condition (kernel/user.php):

if (get_class($user) != 'xoopsuser') {
return false;
}

resulting in the dreaded "Could not register new user" without any further debugging information.

Now to the ranting...

I really missed a FAQ addressing this issue. There are maybe 100+ posts with the same problem! Searching for this message in this (great) site provided hundreds of hits, most of them duplicated (i.e. a hit for every post in a single thread). Really frustrating.

I know, I should be using a newer version of XOOPS, but where I'm standing right now this is kind of hard. So, I leave this info in case it is usefull for someone else.

Thanks!