JQuery error caused by wp-greet

Hi,
I am experiencing some problems with some of my subpages (for example this one: https://www.sophiadeluna.com/index.php/nuremberg/) The admin bar as well as the content (gallery, no text), comment form and footer are missing.
(Example of how it should look: https://www.sophiadeluna.com/index.php/ferns-and-mosses/ … one of the subpages that still works fine.)

Firefox error search says: Type error:jQuery (…).htm. (…) dialog is not a function) … and the indicated line says: .dialog({autoOpen: false, modal: true, zIndex: 10000, title: ‘Subscribe to this blog’}

I don’t understand why this line seems to cause trouble as the exact same line appears in the source code of a subpage that is working.

I deactivated all plugins and reactivated them one by one. Except for the Home page, which has a different culprit for the problems (Smashwords Book Widget), the problem seems to be with wp-greet.

I don’t really know why it is sometimes causing these errors … in the beginning (a few months ago) everything worked like a charm … then it sometimes worked for me but not for others or vice versa or for neither, but usually worked again after a few hours or a day, then worked for quite a while until again the same errors occurred. Now it’s been over a week that the problem persists (no intermitted working any more).

Any idea what I could do?

Help would be greatly appreciated!
Thank you,
Sophia

https://www.ads-software.com/extend/plugins/wp-greet/