since a few days and depending on your exact setup, you may experience this error message when attempting to fetch available system updates:
Repository https://[...] changed its 'Suite' value from 'stable' to 'oldstable'
This is due to the recent release of Debian 11 / Bullseye, and as a consequence, Debian 10 / Buster (which is the current default for YunoHost installations) becomes the new “oldstable” from Debian point of view. This causes
apt update to return an error because the server’s admin but explicitly agree with this small change.
The most straightforward way to fix it is to run
sudo apt update from the command line where you’ll be asked to confirm this change. After this, you should be able to do upgrades as usual (either from the webadmin or command line)