[ceph-users] Failing heartbeats when no backfill is running

Dear ceph-users, I'm having trouble with heartbeats, there are a lot of "heartbeat_check: no reply from..."-messages in my logs when there is no backfilling or repairing running (yes, it's failing when all PGs are active+clean). Only a few OSDs are failing, even when there are several OSDs on the same host. Doesn't look like a network issue to me. When I set the flags "nobackfill" and "norecover" there are no heartbeat issues. My cluster is kind of heterogenous: it's ARMv7 and x86_64, connected mostly via VPN. Some hosts are Debian Stretch, so I'm still using Ceph Luminous (12.2.12). Is there someone having the same issue? What could be the next steps to debug? Any ideas? Thanks for any help! Lorenz