A number of users have reported the search function of the forum doesn't yield any search results.
I can confirm that the search function only seems to search the posts entered after the upgrade. Possibly just a question of re-indexing the whole posts database again.
As a temporary workaround, I suggest forum users use Google to search the forum.
Just enter the following search command in Google's search field to restrict searching to the phplist site:
site:phplist.com your search strings