My apologies but the gallery you are attempting to access has been lost. The thousands of photos themselves are safe.
Please see the following post regarding Cannon School: https://www.kingsquarry.net/index.php/2020/03/15/cannon-school-photo-gallery/
We use NextCloud to host our private family cloud locally on our own server and I was not aware they removed the Gallery app with the upgrade to NextCloud v18 (mid-Feb ’20). While there is a replacement solution called Photos it is unfortunately not able to handle the thousands of pictures covering about six years.
I am working to find a solution but in the mean time if anyone need access or knows of a photo they need please contact me.