I just attempted to register on your forums at stovebolt.com (https), and was successfully brought to "New User Registration" page after accepting "Forum Rules"
My test browsers were:
WINDOWS 10 --
Chrome 66.0.3359.33
Firefox 59.0
Microsoft Edge 17.17123
Microsoft IE 11.0
ANDROID 8.1 --
Chrome Mobile 65.0.3325.109
Firefox mobile 59.0
I had zero issues using those browsers.
(I didnt bother testing with ios/safari or windowsphone/edge.)
Though, when I attempted to go through with
Firefox Focus 4.0.2 (on Android), I was able to reproduce your described error, on stovebolt.com (https)
This leads me to believe that the issue may be related to either a "Privacy Add-on" or "blocker" installed to one of the browsers which are reporting issues.
This could instead be related to "Cookie Path" or "Custom Cookie Prefix" you set for your forums
Control Panel / Primary / Advanced Settings: "Cookie Path" and "Custom Cookie Prefix" (See attached screenshot)
This may also be related to your SSL settings.
As this issue is something I am unable to reproduce on UBBCentral.com, or UBBDev.com (different servers with different SSL settings and different PHP versions), and unable to reproduce the issue on any of the three other servers where I have UBB.threads vanilla installs running, with ALL of the browsers mentioned above, even FF Focus, you may want to review/adjust the items I've listed above, on your own server.
Im sorry that I couldnt be more helpful to you, but I hope to have given you some direction to further debug with.
