I want to echo nextclouda by saying that checking the web console in your browser is very useful. I had the same login button issue. I am using Cloudflare, and I noticed in the web console that CSP was blocking a resource from Cloudflare. I had enabled Rocket Loader (for increased speed!) but it seems that it was being blocked for some reason – probably due to the policies established by the organization I work for. Turning off Rocket Loader solved my problem!