Beware there have been some changes today on the Raspbian repository. They may be moving to Bullseye (although I have not seen an announcement). Updates may not work as expected. I would recommend not updating your Zynthian until we can verify the upstream changes are compatible with Zynthian updates.
It seems that RBPi is starting to move towards next Debian stable. Adding the
seems to solve the problem. It should be fixed now:
Please, update and test.