M2 registration not working

Cannot register, it gets redirected to the same register form, however the URL changes to the following one:

Was able to make it work, once I disabled all the M2 add-ons, and all the plugins except M2.

But after a while the issue popped up, and now switching all the plugins off, and disabling the M2 add-on, and switching to 2016 theme doesn't seem to show any difference.