Sorry anyone’s been having problems with this.
I’m not sure of the exact cause, but the security system that protects us from hackers and spammers automatically turns the alert level up and down according to the level of suspicious activity going on. Sometimes it goes into full lockdown (when there are hundreds of attempts per hour to log into admin accounts etc.) and won’t allow new signups at all.
There seems to be a problem with mobile browsers sending suspicious extra requests with posts etc. that cause the system to go into alert. It can also cause individual accounts to be locked. If you get a “page not found” on the login page, you’ve been locked out. Usually, it would take a large number of failed logins, logging into too many accounts from one IP address, posting a suspicious number of links or posting too fast (eg 5 posts in 10 seconds) to get locked out but some Android and iOS devices seem to set it off more easily.
I’ve tried turning the protection level down but that’s letting spammers get through to post in the forums.