Symptom

Error message indicating that a connection to the SQL database cannot be established. 

Cause

This may be because the database is too large. 

If an SQL Server Express Edition is used, there is a limitation to 10GB currently.

Resolution

Check the directory C:\Program Files\Microsoft SQL (depending on the installation), if the file size is bigger then 10 GB.
Example: 
C:\Program Files\Microsoft SQL Server\MSSQL17.PTVLO\MSSQL\DATA\RouteOptimiser.mdf

If this is the case, it is necessary to delete data and to shrink the database afterwards.