This serialization error is possibly a localization issue, sometimes I have seen this when there are special characters in the string fields that are to be serialized.
Can you look in the mp_TaskQueue table and if the row still exists for this task, can you copy the contents of the SerializedTaskObject field and paste it into a text file and then send it to me at joe dot audette at g mail dot com?
Are you forcing a specific culture for your site by config settings?
What language are your form questions in?
Can you post some examples of your question text so I can try to produce the problem here?