You can’t change your bookmarks right now because they are being synchronized. Wait a few minutes, and then try again.

Open a page you want to bookmark, then try disconnecting from the Internet, and see if you can add the bookmark. Connect to the Internet and see if it continues to give you the error message. Your post is the first post I've read with this problem.

Try running this program in your normal user account, then copy and paste the output in a reply. The program was created by etresoft, a frequent contributor. Please use copy and paste as screen shots can be hard to read. Click “Share Report” button in the toolbar, select “Copy Report” and then paste into a reply. This will show what is running on your computer. No personal information is shown. You can run the report for free at least once, but if you run it several times, at some point it will ask you to pay a license fee.

Etrecheck – System Information

Have you tried quitting Safari and opening it with the shift key held down?

You are welcome. A restart 'resets' a lot of things.