- If you not enter the wrong email + password. i guess there are some bugs in your code and your site are opening the debug mode. So it can't login. So if it is opening => Disable it.
- Else, maybe you need to change the password in database.
I've made some testing on your client site. It is not easy to find out the reason with testing only. It should take time to debug deeply in the code. So if you are interested. Please pm.