@alanfuller You’re right. I forgot about the OP being unable to use the dashboard.
@charic Due to what Alan pointed out you may have no choice other than to do what I suggested in my earlier post and dig through the code to find what is causing this dashboard issue.
However, a few other things did come to mind.
1) You may have a “ghost” htaccess file in the wp_admin directory causing the issue. This was a problem I ran into about six months ago when we accidentally uploaded an updated htaccess file to the wp_admin directory once we deleted the htaccess from the wp_admin directory using the hosts interface (in our case cPanel file manager) the problem was solved.
2) The core wp files which should never be edited may have been damaged which would require you to upload them manually. If you do that you will need to make sure you are getting the files for the same version of your install and then upload them via FTP or your hosting providers interface.
3) Depending who your host is there might be a custom forwarder or cron job in their server hosting panel that is causing this issue. I had this happen once too where it was a custom forwarder added in cPanel’s forwarding feature that was not updating htaccess and still forwarding the admin area to the sites homepage.