Here an update on my situation.
Under more investigation, I found out that the problem is not completely fixed yet. My photo's still look dark in the develop persona, but when I switch to the photo persona it looks right again. If I then switch back to the develop persona the photo also looks good there. So for me the photo's only look dark when first opening a file in the develop persona, after switching to the photo persona it is all good.
Oh and another thing I noticed.
When i open a file, as you know, it opens in the develop persona. For the first five seconds or so of having a photo loaded, the histogram looks like you would expect it to do when looking at the, at this point, still too dark looking photo. Then after five seconds or so the histogram adjusts itself to the correct histogram (what you would expect from the photo when correctly exposed) the photo however remains dark looking. See the video I added showing this histogram thing.
Wouldn't this imply that the problem is not only visual? (affinity doesn't just display the file wrong but it also "knows" it looks wrong according to the histogram?)
The histogram seems to be linked to the way the photo looks (too dark) for a few seconds. Knowing this might be important when trying to solve the problem. Do you get what im saying here, I'm having trouble explaining it.
And o wow, sorry for my inconsistance but now, when opening a file to film the histogram thing, my files suddenly do look right when opening in the develop persona. There is just a short delay as you can see in the video below.
affinity.mp4