Hello all,
I ran into the same issue as all of you.
I found an easier way than reinstall the update via the combo or some of the other manipulations that I have seen.
There is a tool that I've been using for years, and that I actually always recommend and use after an update or even a major upgrade.
It's called ONYX. It helps perform some maintenance tasks such as clearing caches, rebuilding indexes, etc. The only "catch" is that you have to download the proper version of ONYX, depending on your version of MacOS. In this case, we are all having the issue with Catalina so that's a no brainer:
https://www.titanium-software.fr/en/index.html
As I had hoped, after running it and the reboot, everything came back into place. The issue is clearly due to some cache or whatever not being cleaned/reset after the 10.15.7 update.
EDIT: before running it, to help MacOS be a little more at ease CPU-wise, I did follow what some of you did by killing the "Commerce" process, accountsd went from 300% CPU usage to 50%, which is still too high; but at least it gave some CPU time for ONYX --> the scripts and maintenances it launches do require quite a bit of CPU available. Under normal circumstances, the "cleaning" can take up to 10 minutes, so don't freak out.
Here are the settings I used on my side: (in french, but you'll get the english version with the link I provided)
(I don't know if those are default settings or not nowadays, ONYX remembers your past choices and I have been using it since 2008; and since 2013 on this Macbook Air)
It seems like the smoother way to fix our issue.
That being said, *** Apple?
Have a great day,
Jean-Philippe.