NetworkManager fails with "Could not find source connection"

When trying to connect to a VPN using openvpn through NetworkManager, it fails with

Apr 17 02:03:17 GAMINGSTATION NetworkManager[1922]: <info>  [1523923397.5294] audit: op="connection-activate"uuid="186c555a-dd51-4dc1-97c9-70d93c30713d" name="kit"pid=1976 uid=1000 result="fail" reason="Could not find source connection."

However, when using openvpn directly, the connection works.

I found multiple threads suggesting that NetworkManager needs to manage the default network device, but that is the case:

$ nmcli device statusDEVICE       TYPE          STATE                   CONNECTION br0          bridge        connected               br0        enp3s0       ethernet      connected               enp3s0     lo           loopback       not managed            --  

What could be the issue here?

Note: This question looks very similar to Trouble connecting to VPN using network-manager, while command line works, however that question does not have any useful answers.