Well, if what you mean by modifications is limited to adding, editing or removing UBB files then no, this is a clean upgrade. I did add two links to the header for the top menu which I had done in the prior version with no issues, I also added background color attribute to a table tag in the footer. Neither of those edits caused this as I removed the edited files and reinstalled the originals that came with the 7.4.2 zip file.
The custom islands I have are all ones I was previously using as well and they are working as they always have. I did try removing them (hiding) leaving only the Who's On Line island but that had no effect. To summarize, there is nothing different going on when the main forum view is selected than any other page. This is really odd! But everything else seems to be working properly for now. I no longer fear performing updates... it was easy.
Since this is an upgrade performed in advance of moving this web site and forum to a new host and server, should I just perform a fresh install on the new server and import my database and see what happens? It might also be a good idea to re-download the UBB zip file just in case something got corrupted in the process. If you want to investigate this I will fill out a support ticket but I cant do that until I get home and look up my license number.