Dynamics Connector and Dynamics ax 2009 error

one of the maps has stopped working. I've tried to delete the map and restore the map from the test environment but it still fails. The error code is :

[AX Customer to Account] has encountered an error. Processing will be aborted. Object reference not set to an instance of an object.
    --- Exception Dump --- 
    Caught Exception: Object reference not set to an instance of an object.
    Stack trace:
   at Microsoft.Dynamics.Integration.Adapters.Ax50.CustomerObjectProvider.ReadObjectKeys(DateTime modifiedDate)
   at Microsoft.Dynamics.Integration.AdapterAbstractionLayer.MixedObjectProviderProxy.ReadObjectKeys(DateTime modifiedDate)
   at Microsoft.Dynamics.Integration.Service.MapThread.ProcessReads()

WARNING TID:50 [2017-01-25T08:08:13.9049407+02:00]: An unrecognized exception was thrown [Microsoft.Dynamics.Integration.Adapters.Ax50::0x80004003].
INFO TID:29 [2017-01-25T08:08:13.9361903+02:00]: [AX Customer to Account] has failed. 0 record(s) have been written. 0 record(s) have been deleted. 0 record(s) have failed. 0 record(s)

Do you have any ideas what could be wrong?

try to Sync DB and restart AX AOS. I found some reference about your issue: https://community.dynamics.com/crm/f/117/t/173809