Thank you very much, Chowdhury. This explains why the new user account with the default setting for “Disable the visual editor when writing” works fine, whilst the other with the box checked does not.
I hope that me reporting this issue as well helps to get this included in the next update.
Kind regards,
Marcel
22/3 Update:
While testing the admin account I just setup for you, I noticed that I am able to save changes to a textblock again.
It seems like an issue with the WordPress account, rather than the PHP version or other plugin. I am glad that I have found this workaround by coincidence, so my project can move on.
Best regards,
Marcel