Support sent me a solution: you actually need to disable the Gaya simulation scene that comes with the premium upgrade for Msfs2024.
in MSFS Content Manager/My library (2024), search for EGSS
in the search result, you see a line for Premium Upgrade/World Update
to the right of Premium Upgrade, there is a blue 'list' icon, click it to expand all the content inside Premium Upgrade
in the expanded list, find EGSS and uninstall it
then everything is fine.