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Look at the your browsers network console. Most likely something is interfering with the UIs communication with Rancher APIs.

Never mind. The issue was because I put a node on the cluster as a computer node which had a different VLAN configuration. In fact it was a VMware VM being thrust into a cluster of bare metal hosts (naughty naughty!) As soon as I put the VM into maintenance mode, then I can create networks again. A bit disappointing that the error message was so vague and lacking detail though.

Do you have an example error that you'd get from another software? Network Error sounds pretty accurate to a VLAN configuration error.

In that case it should mention something about a "Node network configuration mismatch - Ensure all hosts have the same network configuration" followed by the errors from the log or something like that. I just think having more detailed errors are generally more helpful. In this case I attached a VM to a bare metal cluster sharing the same VLAN tagging. However, at install time, the VM itself did not need to be configured with the same VLAN tag as the bare metal nodes - or it would not work. Hence while the VM worked flawlessly on the network with the bare metal nodes, it's VLAN configuration in harvester was actually different (as in it had no vlan configured).