Just to leave no doubt about this: Nextcloud works without SSL (https).
If you connect without SSL, Nextcloud displays a red warning in the settings menu.
Sorry to open the same thread again but I’m facing the exact same problem.
The login button worked in the first place. When I migrated the data directory from owncloud the login button stopped working.
I have tried the here proposed solution to comment out the instanceid entry in config.php and I have checked that a new appdata_{instanceid} was created in the data directory and restarted apache. However, the login button still doesn’t react and you can see in the browser that the control is disabled according to the CSS properties.
Has anybody an idea how to resolve this issue?
EDIT: Problem resolved! It was really just matter of clearing the cache for the site on the client browser after commenting out the instanceid field.
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!