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=18.104.22.168, Culture=neutral, PublicKeyToken=71e9bce111e9429c' is not marked as serializable. 2b80fdd3-05b4-494c-827a-f1dc8abcd933