Here is the complete error message which my client reported.
Msg 3637, Level 16, State 3, Line 1
A parallel operation cannot be started from a DAC connection.
Msg 3013, Level 16, State 1, Line 1
BACKUP DATABASE is terminating abnormally.
By looking at the error message, I asked him how he is connecting to SQL Server and where exactly the error is appearing.
They informed that they have scheduled a backup job from Windows. Since it is an SQL Express edition, they created a batch file to take backup of the database. The batch file was running as a scheduled task via windows task scheduler. The error appears when a backup is initiated.
I asked them to show the batch file which is taking backup and it didn’t take much time to find what is going wrong while taking backup.
I was able to reproduce the error in my local lab environment as well.
To reproduce the error, I connected to SQL Server via SQL Server Management Studio and gave the server name as “Admin:ServerName”. This caused the connection to go as Dedicated Admin Connection or DAC. This is not a normal connection to SQL Server via 1433 port. There are some limitations of DAC connection and unable to take a backup is one of them.
In my client’s situation, the .bat file had below.
SQLCMD -S <ServerName> -E -A -I”<path to backup script>
When I looked at the documentation, it says [-A dedicated admin connection] and that explained everything. Here are my test results, with -A and without -A and you can see the difference.
In short, I asked the customer to change the script to remove “A” parameter to avoid DAC connection and since then the backups are running like a charm.
Reference: Pinal Dave (https://blog.sqlauthority.com)