I posted about my problems updating from 2020.03 to 2021.09 (10 Oct 2021, 14:11h). This problems are solved for now. Many thanks to @hoergen
and especially @Michael Vogel
for your support!
Lessons learned: "Update each major version step by step
I had to restore my database backup (**), therefore my original post is not available to me anymore. But all other posts are here. I experienced the known and already solved hiccups for updating from 2020.09 to 2021.01 (tables notify-threads and post-category) and 2021.07 to 2021.09 (tables (temp-)apcontact, (temp-)fcontact and delayed-post).
* At least until there is an other update procedure. If you use git, use "git checkout tags/release-name" after "git pull".
** Having a working backup (and restore) strategy is really good to know.