Backups are failing with 2106

Hello Team,

The backups are getting failed with error code 2106 at some point of time as mentioned error below. But when I retrigger the backups the backups are getting successful. Sometime it is failing on second time also when i retrigger but  third or fourth time when i retrigger the bakcups they are getting successful.

but frequently the backups are getting failed with EC 2106.  Kindly suggest on which I need to check.

Error nbjm(pid=2992) NBU status: 2106, EMM status: Storage Server is down or unavailable 
Disk storage server is down(2106)