Hi,
I had the same problem, but only changing the category back in the forum properties didn't help all that much, i could see my forums again, but when u are in a topic, and click on the forum in which the topic is located the page was blank.
I found the solution to this problem:
delete the file "categories.file" in your base UBB directory and recreate the categories and associate the forums with the category, then everything works
Greets
DaViper
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