Its just happened to me in Firefox but I know what I did. I clicked on the expanded view button for the off topic forum and the whole of Off Topic now loads as a blank page
Loads as normal in condensed view in Chrome
Cleared cache and cookies from FIrefox, still the same.
In this forum I can change between condensed and expanded view no problem
The issue appears to be specifically with expanded view in Off Topic
Windohs 10 in case you need that (Work machine at home)
I've now recreated the issue and to solve it, I have to go to another forum, change back to condensed view in that forum and then OT works again.
My conclusion is that expanded view in Off Topic is what is fubarred