Thanks Keystone, I tried that, but the reply was something like "I don't know, If you shared the bookmark list with others, maybe others have deleted it???"... so I don't really see competence in this answer... Also I got the answer, that there is no help if a bookmark list is deleted, it cannot be recovered in no way possible. Additionally, by the way, I found a second bookmark list of mine to be deleted as well:
https://www.geocaching.com/bookmarks/view.aspx?guid=595468b2-5c45-45ca-bc28-c019320533b6
I had pocket queries for all my bookmark lists, unfortunately they didn't run when the deletion happened, so I cannot recover them with that, too :-(((
I am very angry about that topic. I used this bookmark lists for over 10 years now and never had a problem like that before...