I/O is frozen on Database & also I/O was resumed on Database on daily basis in log with complete Backup
I believe its just a message and not an error unless and until the task is getting impacted when this message is logged in the error log:

We also encountered the same for one of our production server, where windows admin team enabled the SQL Server backup through VSS (Volume Shadow Copy Services). And when this process runs it freezes I/O temporarily to take shadow copy and release it back once process is complete.

Major cause being "I/O on the database is frozen in order for any backup service using Virtual Device Interface (VDI)  and commiting a sql  snapshot backup."
as mentioned here

For a better understanding how this process works please read How It Works: SQL Server - VDI (VSS) Backup Resources2

Therefore i believe you should start looking for process like Volume Shadow Copy (VSS) OR Data Protection Manager (DPM) running on the server and discuss with system admin team for what is enabled and to ensure nothing is impacted.