I cannot log in from the front end, but I can still log in from the backend to my admin page.
This page has just been transferred to a new domain name, as I had a 5-month-long break, and rebranding my site as the content ended up being a bit different from my initial thoughts, so a new name and branding are needed for my return.
I have made sure my host uses the same PHP version, I bought and updated all extensions (three-month package for 99 euros), and I changed my site key with the updated domain name. Also, I have changed mysql so the site runs separate.
I am sure there is a basic thing I have forgotten. If so, give me a direction, and I'll try, but if you guys need a superuser and login to my site/ftp, let me know where to send it.