Welcome to discourse @Friso
Further to @ufechner7 requests for info; Have you updated anything between yesterday and today? If yes, what and from which to which version(s).
Given the code works for @ufechner7, it must be something in your environment.
See Please read: make it easier to help you .
Try whittling your code down to a Minimum Working Example that still shows the error, instead of just dumping your entire code. This will a. often help you find the error yourself or b. make it easier for us to focus on the error.