Is this between two Simulator Clusters ? (inter or intra). It's difficult to say what could be the cause, especially on a Simulator, unless looking at the logs. However, considering this is a simulator, one option is to just upgrade to the patch release. I found a kb, that covers a issue impacting InterCluster SnapMirror, and it suggests upgrading to 9.11.1P3. Hopefully, someone who has this version of Simulator, can share the experience regarding this issue.
Cluster peering issues after upgrade to ONTAP 9.11.1 - NetApp Knowledge Base