Your MTU is much too high.
Max MTU for IPv4 only is 1440.
But as IPv6 can be involved the default MTU is 1420 (1412 if you are using PPPoE)
So start with 1420.
This might even be too high, if it still does not work lower it e.g to 1280 or even lower.
Other thing which could be related is IPv6.
You only route IPv4 via the vpn.