Thank you for your reply
I do all of my development on my local system and after testing I upload the site to the live server. I find this the most comfortable way of working.
The Add to Site admin page gives me the following URL addtosite.php
As said, evrything on the local server works fine with the exception of showing the actual gallery.
The Choose Design page has this URL choosedesign.php
Then when I click on Preview the everlasting loading symbol appears. The URL for the preview is index.php?gallery=1&theme=light