I was just comparing two backups against another database. I had restored one database and compared it with Sql Compare from Redgate. All good! Then I restored another database to compare against the same database. No go….
“Login failed for user bla“.
Error ID: was 18456.
Silly me! When i restored the second database, I had given it a name different to the first one!
If anything goes wrong “Check the servername, username, password, AND database name!” or in other words check all the connection string/credentials completely!!!