Hey @JortHoogeboom
Since the earlier version ended up working, but your work is lost, something definitely has changed between then and now. Finding that change has proven quite difficult unfortunately, it may be best to reach out to ESRI Support to see if they are able to find the precise issue that is coming up.
The last suggestion I have is attempt to rebuild the project in another Pro document, relinking the FGDB up to the project to see if that changes anything, otherwise, support would be best.
Cody