If you have a split data centre and require that CAS servers in one data centre can access mailbox servers in the other datacentre you need to enable "AllowCrossRPCClientAccess" otherwise these request get denied if false.
Check its set to False with this.......
Get-DatabaseAvailabilityGroup –Identity DAG
Then update it to "True" with this.....
Set-DatabaseAvailabilityGroup –Identity DAG -AllowCrossSiteRPCClientAccess:$True
AllowCrossSiteRPCClientAccess was reverting back to it’s default value ($False) unless it was explicitly specified in the command.
If you have set AllowCrossSiteRPCClientAccess to $True for a DAG in your environment, please ensure that when running Set-DatabaseAvailabilityGroup cmdlet you specify the AllowCrossSiteRPCClientAccess. You can also run Get-DatabaseAvailabilityGroup to validate the configuration on a scheduled basis.