wp-admin shows blank page, how to fix it?

Try turning on WP Debug. If this is happening due to a PHP error (which I bet that it is), you will be able to see what's going on and fix the error.

After dozens of times trying to fix this problem reading forums and posts, reinstalling WordPress, removing white spaces, putting lines of code in wp-config.php, index.php, admin.php, I fixed the issue just by renaming the plugins folder to "pluginss" in FTP. So wordpress asked me to update the database. I updated and I could enter at /wp-admin. A plugin was causing some conflict, so when I rename the plugins folder, all plugins automatically has been disabled.

As I was inside the /wp-admin dashboard, I could rename the "pluginss" folder to the regular name and start to activate all the plugins one by one and see what plugin was broken.

Now is 100% fine.