I am facing same issue while re-connecting to my TunnelBear VPN. TunnelBear VPN uses open-vpn service. I have configured VPN as per instructions by TunnelBear. And it works fine when connect first time, but after that when choose "Turn Off" from Netowrok-Manager then choose "Connect" again, then I see this same error. See snapshot from syslog:
Dec 17 20:45:25 KUMAR-PC NetworkManager[902]: <info>  [1576611925.5171] vpn-connection[0x5568a73e41c0,d79431f3-7dda-41cb-b74b-e7b71ed48592,"TunnelBear India",0]: VPN plugin: state changed: starting (3)
Dec 17 20:45:25 KUMAR-PC NetworkManager[902]: <info>  [1576611925.5172] vpn-connection[0x5568a73e41c0,d79431f3-7dda-41cb-b74b-e7b71ed48592,"TunnelBear India",0]: VPN connection: (ConnectInteractive) reply received
Dec 17 20:45:25 KUMAR-PC nm-openvpn[6237]: WARNING: --keysize is DEPRECATED and will be removed in OpenVPN 2.6
Dec 17 20:45:25 KUMAR-PC nm-openvpn[6237]: OpenVPN 2.4.4 x86_64-pc-linux-gnu [SSL (OpenSSL)] [LZO] [LZ4] [EPOLL] [PKCS11] [MH/PKTINFO] [AEAD] built on May 14 2019
Dec 17 20:45:25 KUMAR-PC nm-openvpn[6237]: library versions: OpenSSL 1.1.1  11 Sep 2018, LZO 2.08
Dec 17 20:45:25 KUMAR-PC nm-openvpn[6237]: WARNING: --ns-cert-type is DEPRECATED.  Use --remote-cert-tls instead.
Dec 17 20:45:25 KUMAR-PC nm-openvpn[6237]: NOTE: the current --script-security setting may allow this configuration to call user-defined scripts
Dec 17 20:45:25 KUMAR-PC nm-openvpn[6237]: TCP/UDP: Preserving recently used remote address: [AF_INET]103.228.153.233:443
Dec 17 20:45:25 KUMAR-PC nm-openvpn[6237]: UDP link local: (not bound)
Dec 17 20:45:25 KUMAR-PC nm-openvpn[6237]: UDP link remote: [AF_INET]103.228.153.233:443
Dec 17 20:45:25 KUMAR-PC nm-openvpn[6237]: NOTE: chroot will be delayed because of --client, --pull, or --up-delay
Dec 17 20:45:25 KUMAR-PC nm-openvpn[6237]: NOTE: UID/GID downgrade will be delayed because of --client, --pull, or --up-delay
Dec 17 20:45:27 KUMAR-PC nm-openvpn[6237]: [9563/server] Peer Connection Initiated with [AF_INET]103.228.153.233:443
Dec 17 20:45:28 KUMAR-PC nm-openvpn[6237]: Option 'explicit-exit-notify' in [PUSH-OPTIONS]:3 is ignored by previous <connection> blocks
Dec 17 20:45:28 KUMAR-PC nm-openvpn[6237]: Note: option tun-ipv6 is ignored because modern operating systems do not need special IPv6 tun handling anymore.
Dec 17 20:45:28 KUMAR-PC nm-openvpn[6237]: ERROR: Cannot ioctl TUNSETIFF tun0: Device or resource busy (errno=16)
Dec 17 20:45:28 KUMAR-PC nm-openvpn[6237]: Exiting due to fatal error
Dec 17 20:45:28 KUMAR-PC NetworkManager[902]: <warn>  [1576611928.9900] vpn-connection[0x5568a73e41c0,d79431f3-7dda-41cb-b74b-e7b71ed48592,"TunnelBear India",0]: VPN plugin: failed: connect-failed (1)
Dec 17 20:45:28 KUMAR-PC NetworkManager[902]: <warn>  [1576611928.9901] vpn-connection[0x5568a73e41c0,d79431f3-7dda-41cb-b74b-e7b71ed48592,"TunnelBear India",0]: VPN plugin: failed: connect-failed (1)
Dec 17 20:45:28 KUMAR-PC NetworkManager[902]: <info>  [1576611928.9902] vpn-connection[0x5568a73e41c0,d79431f3-7dda-41cb-b74b-e7b71ed48592,"TunnelBear India",0]: VPN plugin: state changed: stopping (5)
Dec 17 20:45:28 KUMAR-PC NetworkManager[902]: <info>  [1576611928.9903] vpn-connection[0x5568a73e41c0,d79431f3-7dda-41cb-b74b-e7b71ed48592,"TunnelBear India",0]: VPN plugin: state changed: stopped (6)

If someone needs, then I can reproduce this with additional traces. But as much I could learn from this problem is that, while "Turn Off" from Netowrk-Manager, Network-Manager is not able to stop openvpn process.
If I stop this openvpn process manually, then try to "Connect" from Network-Manager menu, it works fine.