Sure. Your server is tinkering with Location:

erlangen:~ # curl -I ringelstetter.net/Holy_Land_2013
HTTP/1.1 301 Moved Permanently
Server: nginx/1.14.1
Date: Fri, 19 Apr 2019 14:47:16 GMT
Content-Type: text/html; charset=iso-8859-1
Connection: keep-alive
Location: http://ringelstetter.net/Holy_Land_2013/?where=Holy_Land_2013

erlangen:~ #

vs.:

erlangen:~ # curl -I mistelberger.net/Tiger
HTTP/1.1 301 Moved Permanently
Date: Fri, 19 Apr 2019 14:47:06 GMT
Server: Apache
Location: http://mistelberger.net/Tiger/
Content-Length: 238
Content-Type: text/html; charset=iso-8859-1
X-Varnish: 148082675
Age: 0
Via: 1.1 varnish (Varnish/6.1)
Connection: keep-alive

erlangen:~ #

The folders will take care of that. The link to the album about your dog would be:

<a href="mydog/index.html">

And the link to the album about your cat would be:

<a href="mycat/index.html">

Renaming the index page is an unnecessary step.

In any event, I think Laza is absolutely correct - your pages are being fed through some sort of "page optimization" script before they're served to the site visitor, and that is what's causing them to fail.

ETA: BTW, since index.html is the default name for a page on the web, the link to your album, instead of being this:

http://www.ringelstetter.net/Holy_Land_2013/holy_land_2013.html

Would be just this:

http://www.ringelstetter.net/Holy_Land_2013/