You have a complex situation, due to the superposition of two distinct issues: OpenVPN and NetworkManager. It is not clear to me where exactly the problem lies, so I suggest you activate OpenVPN directly, without passing thru Network-Manager:
sudo openvpn --config /path/to/your/file.conf
If this this works, then we can be sure that the fault lies squarely with Network-Manager, while if it does not, error messages will be provided (either in /etc/openvpn or in /var/log/openvpn.log, depending on how you set this up).
I never pass thru Network-manager because I have had problems with it in the past. Since you are a fellow Archer, one might think that we may have experienced a similar problem, but I think this unlikely. I have never managed to pinpoint it, but I found that a reduction of the mtu helps: say, use mssfix or even mtu-test in the configuration file. However, these options only make sense for OpenVPN over UDP (my case), but if I read your comments right, you are using OpenVPN over TCP, so no dice there.
At any rate, I have resorted to connecting to OpenVPN directly, as per the command above. Hope this helps, cheers.