Thank you for your reply and your feedback.
My website is only for visitors from the Netherlands and Belgium. Only visitors from other countries will see that screen. I whitelisted Google, so GoogleBot will also not see that screen.
This has always been the case and has never caused any problems in the 8 years that my website exists.
As you can see in my first post, Google also doesn’t detect any errors.
So even though that unattractive page appears to you, it is not the reason that my URLs are disappearing from Google.