I’ve got a very similar problem with StarCraft 2 APIs tonight.
/oauth/token endpoint returns error 500 when it receives client credentials encoded as base64 string in Authorization header. It used to work fine until tonight. Node.js example mentioned here doesn’t work for me either.
Funny enough, when I try an equivalent request using curl -u CLIENT_ID_HERE:CLIENT_SECRET_HERE ... to /oauth/token it works fine.
I’m not sure if I should update my API library to use different auth method or it’s just a bug on Blizzard’s end.