@clemylton welcome to the Rockstor community. Thanks for sharing your experience on varied hardware.
You are correct, the Network Manager is a pre-requisite for Rockstor services to start successfully. The symptom you’re seeing is often time related to systems that have multiple NICs installed, and for some reason one of them takes too long to come up and report it back to the Network Manager Wait Online service, leading to a timeout and the consequences you’ve observed. There does not seem too much that can be done about it (aside from restarting the network manager and then subsequently Rockstor) at this time.
In the future there will hopefully be a better way to circumvent this limitation (e.g. systemically identifying if at least one Network interface is up, or something similar).