Somehow, by messing with a spare installation, I managed to simulate this fault. It certainly does have a connection with the "Links"
I removed the Links. When I logged on I log on and opened a page, then immediately tried "and add it to favorites", it worked. As soon as I had multiple tabs, it stopped again.
I moved the 'Links' folder from the "username" folder, to a safe place, together with copying my favorites to the same place. and then, when I tried an immediate 'Add to Favorites", a new Favorites Bar folder was created. I copied all the items from the old 'Links' and favorites folder, and everything was working.
P.S. This may be important! I found, by default, a Links folder actually in the Favorites folder.I deleted this also.