What do the HTML Editor problems mean in the 'Corrections Required' error when I save a message template?

Environment: 

Oracle B2C Service Version 20A+

Issue:

I am trying to save a message template in a recently upgraded site and get an error:

HTML Editor - Case Statements may only contain Conditional Sections and Default Sections. There must be at least one Conditional Section and at most one Default Section.  A Default Section, if present, must be last.

However, I don't get this error in my older (test) site?  How do I fix the problem so I can save my message templates?

Resolution:

This error is often caused by having a Conditional Section within the Default of a Case Section.  A Conditional Section within the Default section or below it was never intended to work, but the ''Corrections Required' error message was not working properly prior to version 20A.

A simplified version of a message template that causes this message is shown with a Conditional section 'Login is null' embedded into the Default of the Case Section.