If you try to use the built-in Forum search function (Find tab) and get an error message, you can use Google instead:
-- a. Open a Google search.
-- b. In the search box, type in <search term> site:forums.ubi.com. For example, to search for patch information, patch site:forums.ubi.com.
-- c. Click on the Google Search box to start searching.

You can also do an Advanced Search. Type in your search term, type forums.ubi.com in the Domain box, and make any other desired changes on this page. Then click on Google Search.

Unfortunately, the results will contain instances of your search term for ALL of the Ubisoft Forums, not just the one you're currently in, but that's better than no search capability at all!