Man, honestly, are you just want to kill your plugin or what?
1. Links you provide is usually incorrect. Well, i can find the correct link.
2. You don’t give an ability to register with plain e-mail/password, ok, i see you don’t care that 3rd party registrations may be deprecated one day or these services disappear/change policies, ok… but it does not work “Not Acceptable!
An appropriate representation of the requested resource could not be found on this server. This error was generated by Mod_Security.”
I would highly recommend you to:
1. Stop making shitty things like “support only here”.
or
2. Take care of the “external” things you control. Hire sysadmin if you’re unsure.