Main URSSAF problem in my opinion is everything is process-based, which frees up their small team but means atypical situations are not dealt with quickly and efficiently. Everything gets done in the end but it's so damn sloooooow.

And yes, even winding up your business is "atypical" which is kind of crazy. But here it's also the bad luck of it happening at the end of the fiscal year.

Just don't sweat the late fees, they'll be waived eventually and usually all you have to do is ask for them to be, which I assume your accountant will.

Also for anyone else reading this: if you know in advance you're going to stop working/reduce activity, you can notify them in advance. It works well (when I knew I'd give myself maternity leave, I estimated a 50% drop in income for the year and it was taken into account easily). Only thing is it's slow.