You have several issues with your site, some comes from phorum.
1) On the create a new account page.
Click in the username box. I get a warning password is not secure.
Inspecting I find that it is linked to an URL that is accessed by http, and not https.
All modern browsers, and the internet itself treat http and https as separate resources with their own domain name.
adding a www . in front of mywebsite . com, is treated by some hosts as a different web site or unconfigured.
You need to go through the code, and change http : \\ to https : \\ every where you can locate it.
If you are on a nix server, write a redirect to https : \\ in the .htaccess file.
In the admin General Settings / File/Path Settings put in ... https : \\ www . mysite/ myforum
ajax and java script are old and have http paths embedded. (can't advise you on the fix yet)
2) The mobile template has not been looked at by a developer in probably ten+ years.
It is missing some parts.
" /mobile/profile.tpl" does not exist "
" /mobile/register.tpl" does not exist "