Thanks for your input.

... but both apt-get and synaptic report 0 broken packages.
You read right.
The thing is that SPM lists 1683 in the 'Broken' filter screen.
Should be none listed there. (?) (see the screenshot)

... may be just an error in the number of packages listed in the broken filter ...
Could be.

The thing is that after a complete removal of SPM and reinstallation, the list survived.
Shouldn't it have been done away with?

... left overs from your upgrade. Just a guess.
Could be ...

But then it could be something else.

What I did find interesting/strange was that in the course of removing SPM (using SPM, of course) I noticed that the number of packages listed by SPM as 'broken' was suddenly reduced to five or six packages. The application dissapeared from screen as I intended to delve further.
Makes me wonder ...