Backups in Availability Groups In case of async commit setup and the backup preference set to secondary replica.
I have observed, in case of high unsent log size, log backup on secondary replica starts failing for that database with error:


  Connection With Primary Replica Not Active


The unsent log size was ~4GB. There was no connectivity issues.

What factors impact log backups in multiple secondaries and async commit mode?
You can backup on any secondary replica if they are connected to the primary replica AND the state is either SYNCHRONIZED or SYNCHRONIZING.

It seems that you should check the connection to the primary replica. Please take a look in the sql server's error log.