An implementation issue (arguably a bug) in how nginx 1.9.x+ negotiates connections, and OkHttp's failure to recover from it, currently prevent the Android app from logging in or editing over an HTTP/2 connection. We've worked around this for the time being by forcing OkHttp to use HTTP/1.1. Meanwhile, nginx is working to modify this behavior in this area and OkHttp is working on a patch to gracefully recover from this connection failure, to be included in the next release (3.3). We should monitor both so that we can benefit from HTTP/2 when a fix is released.
Details here:
https://trac.nginx.org/nginx/ticket/959
https://lists.w3.org/Archives/Public/ietf-http-wg/2016AprJun/0174.html
See also discussion on the OkHttp repo:
https://github.com/square/okhttp/issues/2506
https://github.com/square/okhttp/issues/2543
And related bugs triggered by the HTTP/2 rollout:
{T134758}
{T134759}