The cause of this error is that there was additional work done recently to make variants functionality to be available in the Ecommerce projects.
To fix this, it will be needed to add new Cart element over which all previously created styles and other functionality can be reapplied, and remove old one Cart element from Navbar.
I apologize, for this inconvenience that is caused by changes in the Cart element. We know about this problem with migration of elements, and have internally started looking for the solution, so updating of the element to newer versions will be more smooth in the future.