Why is App Proxy returning error 500 in local development?

[manasan-iTL]

Hi, Shopify!

I am currently developing in a local environment and want to use App Proxy to send requests from Theme to the app backend server.
However, App Proxy is not working well and is returning error 500.

Details
I am using localtunnel to communicate with shopify over https.
As for the API endpoint, I get a proper response when I hit it locally.

Here are the various settings

sub path prefix: apps
sub path: line-connection
proxy url: https://test-manasan.loca.lt/line-connect ()

Theme

fetchUrl: https://development-test-theme-jp.myshopify.com/apps/line-connection/ 

Plese give me some solutions!