Just upgraded from Nextcloud 19.0.10 (2021-05-15) => Nextcloud 20.0.13 (2021-10-10) => Nextcloud 21.0.5 (2021-10-10).
Now all “basic” apps are gone, i.e. no calendar, no tasks, no contacts.
Neitght from web-interface I can install this apps. It look that the AppStore is not available.
From the commanline, it is also not possible.
E.g. php ./occ app:install contact failes.
2021-10-11: In meanwhile I update to Nextcloud 22.2.0 but no improvement - still no appstore access.