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.
Solution:
Silly me! When i restored the second database, I had given it a name different to the first one!
Lesson:
If anything goes wrong “Check the servername, username, password, AND database name!” or in other words check all the connection string/credentials completely!!!
Randomos Technos 
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