Hi Henning,
I still see the old version numbers on your site, as I have mentioned above.
Could you please try to installing and running Transient Cleaner (https://wordpress.org/plugins/artiss-transient-cleaner/) and re-test your issues again.
1. Install Transient Cleaner
2. Navigate Dashboard -> Tools -> Transients
3. Set Clear Expired Transients -> Enable
3. Set Clear Expired Transients -> Clean Now
5. Set Remove All Transients -> Enable
6. Set Remove All Transients -> Optimize Afterwards
7. Set Remove All Transients -> Clean Now
8. Save Changes
9. Recheck those problematic pages
If not, we require your login credentials in a secure note to investigate this issue further.
Thanks!