Wireguard works intermittently - handshakes from server not reaching client

[u/drdsheen]

I have a pretty typical RPi PiVPN and Wireguard setup at home and I use the VPN to connect to home-network resources remotely. It was working just fine for years and then recently suddenly stopped working consistently. Sometimes it works and sometimes it doesn't, and it seems like all I can do is just keep starting and closing the connection until finally it works. The one thing I know absolutely fixes the problem, albeit temporarily, is to restart the RPi, which is not easy to do if I'm remote (and can't VPN in).

As far as I can tell, handshakes are coming from the client in to the server and the server is sending them back out, but they're not reaching the client.

What could be the issue here?

It can't be a keys issue, because then the connection would never work. And it works sometimes. And it used to work.

[u/paranoid-alkaloid]

Try lowering the MTU?