I’ve been seeing these a lot (or at least it seems like it). The URL used to retrieve a vBulletin searches will not work after a short period of time. If you really need to link to a search of the SDMB, use Google Site Search (instructions in this thread).
Note that this does not apply to the pages you find during search, but if you don’t want people to know what you searched for, remove that part from the URL (usually anything following the ? ).