I’ve just had the very same problem with a friends website, this thread helped put me in the right direction so here’s what I did…
1. Logged via ftp
2. Download the .htacess file from the root directory
3. Make a copy, then open the original downloaded file in a text editor
4. Delete all the comments associated with AIOWPS plugin from #AIOWPS_BLOCK_WP_FILE_ACCESS_START to # END All In One WP Security and save as .htaccess (make sure you keep the backup of the original file just in case)
5. Back in ftp, navigate to the plugins directory
6. Disable AIOWPS, you don’t need to delete it, just rename the directory, I put a – in front of the name so it was -all-in-one-wp-security-and-firewall
7. In ftp navigate to the root and upload the edited .htaccess file
8. You should now be able to login using wp-admin eg https://www.yourdomain.com/wp-admin
To restore AIOWPS, in ftp put the plugin directory back to its original name, all-in-one-wp-security-and-firewall, In the wordpress Dashboard, go to Plugins, then Activate AIOWPS, log out of WordPress and navigate to your custom login url eg https://www.yourdomain.com/custom-login/ and you should be able to login as normal.
I’m not sure if this has completely fixed the issue, but for the moment, all seems OK and I hope it helps.
Ohh and it’s WP version 4.3.1, not upgraded to 4.4 yet.