I encountered same issue but i found out that there was some configuration pending deployment, I was able to resolve it by deploying the pending configuration on FDM. Thanks. 

Has any one used the new firepower migration tool (From ASA to Firepower) version 1.2.3? I have been trying but at the last stage it gives me the MTU error. Migration StatusReason for Failure: Error while pushing: Invalid Value for MTU, Allowed range...