Symptom:
Devices that failed a scheduled deployment remain locked, and no further deployments can be done after that.

Conditions:
The possible reason for this defect may be connection pool getting exhausted, because as per the log there was a NPE for a connection object which can be observed in three cases :- 
1) DB shut down  -- Not the case. 
2) Restore DB being taken places -- As confirmed by customer not the case.
3) Connection pool exhausted. 

For this defect 3 case is possible.