Oauth request fails on the test server with "Internal Server Error".

I encountered similar problem and dug down to find out origin of the problem.

There may be two type of origins:

1. Redirection. Protocol of dev server have been changed to HTTPS from HTTP. The DEV server returns 301 (Moved Permanently) response for HTTP request, but some framework doesn't handle it.
2. Certificate hostname mismatch. HTTPS connection does confirming identify of the target server as well as encrypting contents. Basic confirming procedure is done by comparing hostname with commonName field in the certificate. The DEV server utilizes virtual host. It means that the server's action will be different for each requested domain. For this scheme is working, it is needed to send domain name when you retrieve certificate. However, some old SSL library or languages doesn’t have such feature. The DEV server returns a certificate for ‘apis.openstreetmap.org’ when no domain specified and identifying procedure fails.

Solution:

1. Replace http:// to https://. Note that it will introduce the second problem.
2. If there is no change in server then you can do two things – 1) Upgrade your framework or language with latest version. 2) Disable identifying process (I checked some framework and languages, but they didn’t give this option. You may modify core library manually) However, I think that it would better to hand it over to OSM developers.