HI Isaac,
I am using shared hosting, so I've no idea what they've done. I can only tell you that it is a Linux variant.

It is curious though that when I installed it on Xampp just a couple of hours ago, I had the exact same issue on a Windows PC.

Whilst playing around with UBB adminCP settings, I have found that I can use Flat mode in my profile as long as I set the default mode to Threaded.

(If you are experimenting, use a different browser after changing from flat to threaded and vice-versa as browser caching confuses the issue.)

Last edited by driv; 04/26/2019 3:02 PM. Reason: Added the Windows bit.