Default schema is ignored on SELECT
This article gave me the hint:


  It is important note that if the user is authenticated by SQL Server as a member of a group in the Windows operating system, no default schema will be associated with the user. 


My user myuser was a member of the Windows Administrators group and
there was an SQL Server login defined for the Administrators group.

After removing the user from this group, everything works fine.

I've read this documentation before, but skipped the crucial paragraph.