== Solved ==
Update: fixed it. The issue was caused by 2 lines of code in my config.php file.
**== Solved ==**
I am working on this website for a family member. The website is okay for visitors performance wise, but always when I go to the wp-admin section it takes very long to load. The same for when navigating from one are to the other in the WordPress backend.
Now I read about ways to make it faster, and one way is to update the php version. So I went into the hosting providers system and updated from 7.4 to 8.0. But then the website gave a whitescreen of death. Both backend and frontend.
What is the best way to figure out what causes this?
Hope someone can give me a tip to fix this in a smart way.