Just suggestions:
- Open the inspect tool and go to the network tab. Enable persistent logs and try to login while having the inspect tool open. Maybe it can tell you how it redirects so we can get some clue?
- Try removing web browser cache and try again. Chrome has some heavy forced caching. Try Firefox for log in to see if this is a client cache issue.
or if you have access to root;
- restart apache/nginx server and see if this help.