This error took place as part of the default upgrade from Leap 42.3 to Leap 15.

The mail tools updated to a version that needed libxapian22.  The version of
xapian and libxapian was updated to 3.0.  As a result, mu and related processes
could not work.

I re-built mu based on cloning the master git distribution.  This operates with
no problems.  It looks like a simple package building, version mismatch issue. 
This bug should be forwarded to whoever is responsible for maintaining the
Opensuse packaging of mail tools.