Cannot add to favorites

I'm trying to add a new favorite, but a tiny error window comes up: it contains a yellow triangle with black exclamation mark and an OK button, but no other message or symbol. I click on the OK button and another of the same little window comes up. I click on OK again, and it brings me back to the favorites window. My new favorite has not been successfully added. Have I reached my limit for the number of favorites I can add? What should I do?

Thank you for any advice you can give.