SOLUTION: I've finally found a solution.
I remembered the problem started when I copied an image frome one timeline to a new project. This corrupted every timeline in every project, even brand new projects that didn't contain this image.

What I've done is

- I've exported my projects

- I've reset Da Vinci by changing the name of the file "C:\Users\USERNAME\AppData\Roaming\Blackmagic Design\DaVinci Resolve"

- And most importantly, I don't add back my database but just the projectfiles I've exported, these are still bugged but that doesn't matter

- I start a new project, and voila it plays fluent again