

How did you arrive at a 1300B MTU for your openvpn server?
1460B is the MSS for TCP over IPv4 over Ethernet (which has a standard MTU of 1518B) but I can’t figure out the 1300B.
Maybe AirVPN is fragmenting your packets to fit the 1300B, and when the AirVPN server doesn’t bother you get the message too error. Maybe a limitation on individual servers, which is why it sometimes works.
There’s at least two things electricians, mechanics, engineers, and blacksmiths have in common: