VERIFIED SOLUTION i
Appears to be an issue in PD 6.0 through 6.1
If you Create a template in Portrait Dialogue (PD) that uses customSQL and uses one or more fields from the customSQL as merge fields, thumbnailing fails.
This is a bug (Bug 33830: Thumbnails does not work if the message template has a CustomSQL)
ResolutionUPDATED: March 30, 2017
This bug (TFS: 33830 - Thumbnails does not work if the message template has a CustomSQL) is fixed in PD 6.1 SP1 ( PD,6.1.1,Full_18.104.22.1689 )