Weird reverse proxy error on Immich 1.88

I've just updated Immich to 1.88. Followed the instructions and everything works fine ... except my reverse proxy is screwed. When I try to sign in to the server on the app from outside my home network (testing on mobile data) I get a message saying that there was 'a handshake exception with the server', asking me to enable self-signed certificate support in the settings (this option will not turn on though). Bizarrely, it works fine from outside the network using a VPN. I've checked my Nginx setup, have renewed certificates, and have even set up a new proxy link, but still get the same error. All of my other Nginx services are working absolutely fine. Any ideas on how I can fix this?