My best guess is you upgraded this site from a much older version of mojoportal. We changed the way tasks and index items are serialized into the database. At one time we were using SOAP but theSoapSerializer doesn't work in Medium Trust so we switched to plain XmlSerializer.
From the error it seems there are old rows in the db that are still in SOAP format. I would delete all rows from mp_TaskQueue and mp_IndexingQueue, then rebuild the search index.
Hope it helps,
Joe