When plugin wagons are missing from the manager the snapshot restore fails.
This has been tested with the openstack plugin from source on 3.4.2 restoring to a 4.0.1 manager. The snapshot restore stayed executing until the manager was rebuilt.
This problem can be worked around by:
1. Installing the required plugins
2. Restoring the snapshot
3. Re-installing the required plugins (as the snapshot restore will have removed them).
Note: CLI needs updating so that -t documentation is at least correct and preferably so that it fails when -t is used.
The original issue was actually caused by missing dependencies, and exacerbated by us not reporting what's happening during the snapshot restore workflow (unless the user tails the log file on the manager).