This is due to the agent taking longer than the snapshot timeout setting is set to, to take a backup of the disk. This can happen or very large partitions or disks with slow I/O.
This error is common on first snapshots especially because during the first snapshot AppAssure for Linux Agent creates a Copy-On-Write data store which pre-allocates disk space so that when a snapshot takes places, all writes meant for the disk are sent to this storage area until the backup completes.
Adjust the snapshot timeout value for the failing agent.
Click on the Agent in the Core Console and then go to Configuration>Transfer Settings>Change and adjust the following settings to the below value:
Snapshot Timeout: 59:00
Transfer Timeout: 59:00
You do not have to change the value to exactly to 59:00 minutes however in this case you will need to test an experiment to find the best time needed for the snapshot to take place.