After all my conspiracy theories are not that wrong (not when I have all the right info, and I'm not lied nor mis-informed dis-informed)
As I thought, there is a problem with PHP files (hacked by spammers?) in which emails are not sent when a user registers, but I couldn't get Godaddy to admit it. They are the ones who provide PHP, and you can't change it, although it seems you have your own files, as the others sharing the machine yours are in. So they created a control control panel thing for you to fix it.
I kew it! It wasn't our fault!
After my many attempts to fix it, even non-published experiments, lots of blogs and forums read… I found the solution, and ADMITTED by the hosting, IN WRITTING! (I could have solved months ago if they admitted it, before they setup customer access to it, before their legal dept. gave the ok to admit it, long after others ALREADY knew the issue was corrupt PHP files…)
I'm confident enough to say I found the solution for 90% of unsolved or unexplainably solved cases :)
This one FIXES the problem. Not a "install this plugin" article.
So here it is.
It came from Godaddy's mouth but could apply to many hostings.