Items in my sidebar disappear after a few days

I cheered too quickly. Your solution worked for ten day, and since this morning I am back at square one. Only 9 items remain in my sidebar, instead of the approx. 20 I regularly use. So, I think the com.apple.finder.plist is corrupted again.

Something is doing the corrupting, do you have any idea how I could deal with this?

When I migrated to my new laptop I did a fresh install of the system software and all of the applications I use.

This failing sidebar is such a nuisance. By the way, I also noticed that I cannot jump from a folder to its parent folder by way of command-arrow-up, when the folder is based on an alias in a self-made "sidebar" (a folder with links to regularly used folders). If I do command-arrow-up when in such a folder, in the Finder (!), I am transported to the top level of Macintosh HD.

In the Terminal the folder structure is normally accessible with % cd ../

This is the only thing I did not do before, and it sounds plausible, though when I migrated my old laptop to my new one I did a fresh install of the system software.

I now deleted the com.apple.finder.plist and added some silly folders to my sidebar to see if they will stuck (even if I never click on them).

Thanks Leroy.

This solved my issue. Thanks, Leroy! I found I had already one deleted the com.apple.finder.plist but did not then also force quit the Finder. That did it.