I guess this happened after WordPress version update.
Try to deactivate all plugins manually (through FTP or phpmyadmin) cause one of the plugin may cause the conflict.
http://www.wpbeginner.com/plugins/how-to-deactivate-all-plugins-when-not-able-to-access-wp-admin/
active plugins in database already as described in the article and renamed plugins folder to plugins_old
No effect whatsoever 🙁
Try this steps:
1. Log in to FTP or cpanel
2. Downgrage to WP version manually,
Link on how to do this: http://codex.wordpress.org/Installing_WordPress
3. Log in to yourdomain.com/wp-admin
4. Upgrade to WP latest version
Or go to your FTP and try to find file upgrade.php. If you failed to find it, download the latest stable version of WP from their site. Unzip it and relocate to the wp-admin folder.
Or re-uploade all files & folders – except the wp-content folder – from a fresh download of WordPress.
You can get this links because of the missing upgrade.php file. Am i right?
No the upgrade.php file is there…
Also did a manual upgrade by replacing the root files and wp-admin and wp-includes to the server but still the same problem…
I have resolved this error myself! hehehe
I renamed the .htaccess file and renamed the wp-content folder.
after that I was prompted with the notice that the database should be upgraded. somehow on the old server before the transfer someone screwed up an update and left me with the mess.
after the database upgrade which was ready in a jiffy I renamed the file and folder back to the original names and all was working again.
I doubt if it is nescesary to rename the .htaccess file, but since I did I mention it to be complete in my information
