I had to check the version codes for this one, then I remembered. The support of that was removed back in version 2.x, about year and a half ago when BuddyPress converted forums to BBPress internally.
The reason BuddyPress changed to BBPress was that the old forums weren't custom post types, they were a separate data table with custom functions and stuff. Most plugins could not communicate with it, as there was no documentation or interface to make it work. I remember it took me a good while to find out stuff from the BuddyPress core.
Anyways, I don't know if I can restore that to the current version for you, the current Ajax Search Pro core is very different from version 2.0.
I think there is an option to convert the old forums to BBPress, that way the search will definitely work.
If you can't do that, I can look into restoring this feature somehow, but it might take a while depending on how much bug related tickets will I get in the next couple of days.Best,
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