I’m receiving the I/O is frozen on database error :

I/O is frozen on database DB_NAME. No user action is required. However, if I/O is not resumed promptly, you could cancel the backup.
I/O was resumed on database DB_NAME. No user action is required.

What causes this error message?

I/O on the database is frozen in order for any backup service using Virtual Device Interface (VDI)  and commiting a sql  snapshot backup.

Examples of programs using this method are: Volume Shadow Copy (VSS) and Data Protection Manager (DPM).

The aim of the sql snapshot backup is to create an instantaneous copy of the data.  This allows consistency with all the files in the snapshot backup sequence.

From a backup and restore perspective – a SQL snapshot backup holds the same functionality. It is possible to use a SQL snapshot backup as part of a SQL restore sequence with other files that are not snapshot backups.

To check if a backup is a snapshot , check the is_snapshot column on Backup history for a single SQL Server database

Applications should be designed in a way to manage IO coming in . With the understanding flushes to disk  are restricted during the snapshot backup.  The ideal time to execute a snapshot backups is out of business hrs

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