VERIFIED SOLUTION i
Product Feature: Message/Master templates
'about:' is appended at the beginning of the 'View Message in a Customer web' URL while switching from 'design' tab to 'HTML' tab in a HTML Message Template.
This has been identified as a defect in Portrait Dialogue releases prior to 6.2.
ResolutionUPDATED: December 11, 2018
The has been fixed in Hotfix 3 of Portrait Dialogue v6.2.