Something changed that made that "Ye olde reliable" source went broken.
From now all string expressions like "_" inside Sql expressions have to be changed to SqlId("'_'"). With this everything works as expected.
I am sorry about this, frankly this is the biggest flop I have made in years.
The primary motivation was adding quites to column names, but it has gone bad, as many places in U++ code and more importantly client code make assumption about what can and cannot be done with SqlId... So it is now only optional, but still cases a trouble here and there (because I want to keep quotes as option so I cannot remove it completely).
Hopefully, this one will be the last issue... It is fixed and coalesce and SqlVal::As (which was the real cause of problems) is added to SqlExp unit testing package.