Quite silly what happened...
In Administration, i went to the Customer Settings and i cleaned the my account fields (john smith + johnsmith@mywebshop). For some reason, i was doing this so a customer wouldn't have John Smith already filled in a field. From the moment i hit save, i was logged out and ever since, i cannot log in again (with my admin credentials).
Anyone knows how to solve this?

if you have access to the sql backend you can run an update query on the customers (?) table and update the deleted column from 0 to a 1 (or vice versa)

sorry am writing this off the top of my head, to give you a quick reply

Thanks for the answer. I didn't really find how to undo the deletion in SQL table.
Instead I started from another basic shop and copied the necessary files.