When I try to use Linkedin to login to my site on Google App Engine I get a 999 error. I think it must be blocked there because on my local machine the login does work fine.
Some other sites on app engine seem to have the same problem. My only conclusion is that the ip range of app engine must have been banned by linked on purpose or by accident. I think it must be by accident because of how many sites this must affect.
I do not think that it is only related to the Google App Engine, but rather kinda strict blocking policy at LinkedIn. Check out this post here: 999 Error Code on HEAD request to LinkedIn
It seems that LinkedIn also blocks request based on user-agent.
and HTTP Error 999: Request denied and How to avoid "HTTP/1.1 999 Request denied" response from LinkedIn?