Activate plugin without it blanking out wp-admin

EDIT: It’s been fixed, stupid little line of excess code in functions (jquery deregister).

I’m working on a wordpress site for a client, with a theme made from scratch. A couple of days ago, whenever I tried logging into wp-admin (after entering username and password in the login form), I got redirected to a white page, wp-login.php in the address bar.

I just found out it’s contact form 7 that’s doing this (the only plugin installed, but a very necessary one). If I manually deactivate it through ftp, I can get back into wp-admin. Activating it inside wp-admin still allows me to navigate through wp-admin but when I logout, I get the same issue with logging in.

Now I put quite some time into an important form that I need to use, so switching over to another plugin would be a last resort. How can I get contact form 7 active without it blanking out wp-admin after I hit the log-in button?

Just to be clear, I had contact form 7 active a week or so before, and everything worked fine then, no clue what I did to break it.