Edit: After half day, my wife’s Mac started having the same problem. I have then applied the solution that I found in the Apple
Import or Export Bookmarks from and to Safari
If you use several browsers, you may want to share your bookmarks between them. Fortunately, Mac OS actively supports the drag-and-drop feature; therefore the bookmarks can be just dragged from Safari to another browser and vice versa. All you have to do is open both Bookmark windows and drag bookmarks from one window to another.
However, if for some reasons you don’t want to drag and drop your bookmarks, you can go to Safari’s File menu and select either Export Bookmarks or Import Bookmarks.
Clicking Export Bookmarks will create an .html file which then should be imported to another browser. Likewise, you’ll need an .html file exported from another browser to import it to Safari.