Symptoms

Error when filling out an InfoPath form:
"An error occurred accessing a data source.
An entry has been added to the Windows event log of the server.
Log ID:5566"

The InfoPath form controls are not being populated when upon initialize. These controls are mapped to smartobjects
 

Diagnoses

After looking through the SharePoint logs, the error was determined to be caused by a smartobject list method:
"The following query failed: :SMO_Method]...
Item not found in this collection. The remote server returned an error: (500) Internal Server Error."
We then executed the smartobject in question via smo tester and saw that there was an inconsistency with the mapping.

 

Resolution

There were several smartobjects which caused this error. After recreating the smartobjects to update the serviceobject mapping, the error on the tester as well as on InfoPath was removed.