Default Approval WF Unable to Start: CoercionException - Unable to serialize workflow

Question

We receive the following error in the SharePoint logs when we try to start the OOB approval WF from within a SharePoint Designer WF. The odd thing is, the WF will function perfectly, no errors at all, for a certain period of time. Then, without being touched by anyone, will begin to fail.

03/02/2012 09:10:54.75 OWSTIMER.EXE (0x0B84)
0x0C38 SharePoint Foundation Workflow Infrastructure
g7ih Unexpected Unable to serialize workflow, exception: Type 'Microsoft.SharePoint.WorkflowActions.CoercionException' in Assembly 'Microsoft.SharePoint.WorkflowActions, Version=14.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c' is not marked as serializable. 2b80fdd3-05b4-494c-827a-f1dc8abcd933

Answer 1

This was caused due to an issue with a lookup field to another list. In SharePoint Designer a series of 5 columns were being lookedup from list-b via the workflow in list-a and used to build an unordered list in HTML for an email. The values were returned as LookupValue (text) and stored in a string variable. We are not certain why this was occuring in this workflow as there are several other workflows in this list (list-a) that lookup the exact same columns (though from different list items in list-b) and use them in exactly the same way. In fact this exact workflow did this successfully until we changed it to use an approval workflow rather than just create a task and send a one time email.