I have a new “empty” installation of v 3.1 and if I try to register a new user I get the message “The journal is not currently accepting user registrations”
Under Distribution / site access I have ticked “Users must be registered to login” and “Visitors can register user accounts with this journal”
I have had that once, but when I reinstalled it was OK. It is important to put ion php.ini at least 180 seconds for execution time so the OJS have plenty of time to be properly installed.
My suggestion is that you should try to reinstall it.
Please take into consideration that you should use PHP 5.6.30 or higher version.
Good luck.