Network issues on Ubuntu with OpenVPN. Stuck in Synchronizing.

[SomberNight]

Originally reported on reddit

I'm running Ubuntu 16.04 on a VirtualBox VM and using Private Internet Access as my VPN. If the VPN connection is enabled, Electrum won't connect. I get the blue arrows in the corner and it's stuck at "Synchronizing". If I disconnect from the VPN and start Electrum again, it connects and syncs within seconds. With the VPN connected, I can surf the internet, so the VPN connection appears to be good. I also took one of the servers that I was connected to when not on VPN and explicitly set that as my server when I was on the VPN and it still didn't work.

I've managed to reproduce it, on Ubuntu 18.04. (also inside a VirtualBox but I don't think that should matter)
I've tested with two different VPN providers: PrivateInternetAccess and NordVPN.
I've tested with both their custom proprietary clients; and also the stock OpenVPN client (2.4.4) and .ovpn files.

(Also tested on Windows 10, which was not affected.)

The issue occurs when using the default settings, with protocol=UDP and MTU=1500.
Both providers with both their custom client and stock openvpn are affected.

If I change the protocol to TCP, or keep UDP but lower the MTU size a bit, the problem disappears.
(so e.g. setting either mssfix or tun-mtu to 1300 works)

This does not seem to be something we can/should do something about.