macOS Monterey disables brightness controls on Apple 24" LED Cinema display

On Sunday I upgraded my 2020 mac Mini M1 to Monterey and the display brightness controls stopped working. Both the keyboard special keys and the "display" slider in the macOS menu bar are disabled. All other features of the Cinema Display, the sound control keys and all other special keys still work as they used to under Big Sur.

This is a very serious problem, as health & safety regulations mandate that any computer display must be equipped with brightness controls, that mac mini is now not suitable for any production environment.

Does anybody know if this is recoverable?

(Moreover: is this a known problem? Neither the upgrade procedure nor the docs alerted me to this beforehand. )

Best regards,

*MM*