Brian, on first glance it looks like you've had this exact problem since at least September 2021.
Prior to that, you had issues with your hosting company (hostbaby) since January 2020.
Did the issue first present itself right after a software upgrade? Did you replace all the files as required?
It could be possible that you are also attempting to load a severly outdated library, plugin, or utility on the server backend. possibly even a server scripting configuration or htaccess setting.
Log in to your hosting account and restore the configuration settings to those of when your webpages were displaying correctly. My advice is to contact your webhost for support with this unique issue.
Your installed versions:
UBB.threads 7.6.0 (released 2017)
Smarty 2.6 (PHP 5.4 to 5.6)
UBB.threads 7.7.5 (current)
Smarty 3.1.36 (PHP 5.4 to 7.4)