Is there something that I am missing?
Newly created database with 2 users. Both have the same permissions. SQL01 gets error "The database DB01 is not accessible" on browsing through SSMS.
No login failures in SQL errorlog.
No user error message thrown in a profiler trace.
user's default database is set to master
SQL01 can access other database on the same instance.
USE DB01; --throws error
The server principal "DOM\SQL1" is not able to access the database "DB01" under the current security context.