We just put out 2.3.5.103 with a fix for failed migration message and associated Zigbee problem. In a nutshell, if you got that message, Zigbee didn't migrate although database and Z-Wave did.
If you've shut C8 and reverted back to running C7, running 2.3.5.103 on C8 and attempting migration again is the way to go. Make sure to take a fresh migration or cloud backup on the source hub first. Zigbee keeps frame counters; those need to be in sync between the hub and the devices talking to the hub.
If you've already added Zigbee devices manually on C8, no further action is needed. Trying to migrate again will only result in having to add devices again while cursing at the hub's developers.
