UBB.threads PHP Forum Software Community Forums UBB.threads Bug Reports [NotABug] .swf

Adobe Flash from last decade
https://en.m.wikipedia.org/wiki/SWF

It's probably displaying for 1 of 2 reasons:

1/ a rogue ad that you are displaying.
or
2/ a program/malware the user has installed on his computer.

Have your user Google their problem. This has nothing to do with UBB.threads. It is not a file or file format that ships with the forum software.