Development Environment - Admin Connection Not Working?

[BSS98]

Hello everyone, 

I have looked everywhere before posting this, but could not find a topic for this specific case.. I hope I did not oversee anything.

I am on PS 1.7.7.4 and installed a copy of my current website under a /dev/ subdirectory, to make changes and then push them live when I am happy with them. After working on making this and managing to connect the files, db and editing what had to be edited (thanks to this tutorial mainly), the Front-End is identical to the main website, but I have now come to the realisation that I cannot login to the backoffice of the website.com/dev/ directory. 

I am using the same admin connection link than the main website (as I copied the files I assume the link remains the same) and the login form appears correctly. I then proceed to type my login details but the page just refreshes and resets all values to blank. I have tried to type the wrong password and then an error message appears, because I get no error message when typing login credentials correctly.

Does anyone know by any chance why this happens? I have checked the error logs but nothing regarding this appears, and it seems that my .htaccess file is configured correctly. 

I have tried private browsing etc and it does not seem to be an issue like this, I have also reset the password but the new one does not give me access either and just triggers the same thing. 

Any insight on why this happens would be appreciated. For information: this only happens in the /dev/ admin login page, not the main website's login page.

Many thanks!