Just restarted my service and the issue persists.
I tried including my own auth_token in the request, and I instead receive an ERR_BADAUTH response (expected behavior). Upon changing the auth_token to one linked to the owner of the channel, there are no errors.
It seems like the Server Error response only happens when the auth_token field is omitted from the request. I don’t know about you, but I feel like this is a bit misleading, and the error message should be changed to ERR_BADAUTH (or possibly ERR_BADMESSAGE) on Twitch’s end.