I found a workaround:
apt install chromium/bullseye chromium-common/bullseye
rm -Rf /{root,home/*}/.{cache,config}/chromium
This will downgrade Chromium from v98 (from Debian security repo) to v90 (from regular Debian repo). That one starts up without issues.
Any next “apt upgrade” incl. “dietpi-update” will upgrade it again. To lock it:
apt-mark hold chromium chromium-common
I’ll report this to the Debian bug tracker.