Snapshot isolation transaction failed in database 'Master'
Fortunately we have trace running in production.
Analysing the data I found an 'ALTER LOGIN' being doing in the 'MASTER' system db.

But it still strange because those commands are from a DIFFERENT APPs using a different connection, technology, etc.
Basicaly costumer got an "COA - Client Online App" and support have an "ALA - Adm Local App".
Locks like support changed the password of a user in the ALA and caused the COA to raise a error in a DB intensive operation.
It's still strange because even the users/costumers in both instances are different and don't share any data.

I was naively expecting two unrelated transactions from two unrelated connections to not crash one another.