I have the same problem, and I used the SQL executioner, deleted the row but it still appears under the forum thread as a post, but after I click on it, gives me the 403 error.
if I try to delete it again with the SQL, it says, 0 rows were affected. So it did delete the row but I guess the entry in the forum is still there.
Any ideias?
I had this exact problem yesterday. One of my moderators via the dashboard moderator tool, accidentally deleted the first post of a topic.
Going into the forum as admin to look at the topic, what was now the first post did not have the usual "first" post options. You could no longer "move" or "make" the topic a "sticky" etc.
Now I would have thought what was originally the "second post" but now the "first post", would have inherited the status of "first post", thereby having all the "move", "sticky" options available on it.
As the topic was of little importance I deleted the remaining posts that were left in the topic, fully expecting that when the last one was deleted, the topic would be gone from the forum but it was not!
Going back into the category, the topic title was there and its status was "last updated 48 years ago!!!!". when I clicked on the topic I got a "404 error". I thought OK I guess I need to flush the caches via the dashboard. Did the flush, it did not help at all, the "phantom/zombie" topic was still there.
I then went in via phpadmin SQL tool via my c-panel. Located the wpforo "topic" table and then using a "select" statement "where" "title" was "like" the topic name, I found the "phantom/zombie" topic.
I then deleted the topic from the database table. Flushed the cache again via the dashboard. Now when I went back into the forum the "phantom/zombie" was now gone.
Just thought I would mention all this in case anyone else has a similar problem and finds it useful.
Warning: Be very careful in the phpadmin SQL tool, manipulating the Db can have the potential to really screw up your database if you get things wrong, if in doubt leave well alone.
