I assume since the last version of contacts got installed, the contacts app is empty for all users. DAVx seems to synronize without errors, so I must assume that the contacts are still stored in the DB but the web app cannot read the information.
The last 5 days I had not problem with the contacts app loading, any more. Seems that the initial error
2023-02-27 13:22:59 650 [Warning] Aborted connection 650 to db: 'nextcloud' user: 'ncadmin' host: 'localhost' (Got an error reading communication packets)
2023-02-27 13:22:59 651 [Warning] Aborted connection 651 to db: 'nextcloud' user: 'ncadmin' host: 'localhost' (Got an error reading communication packets)
Gets solved by @szaimen idea of raising the max_allowed_packed value. I raised it to 256MB but I cannot say if it is too much or not enough.
I am still wondering what has changed from the previous contacts web app to the one raising the error in my setup. Perhaps this could be checked by nextcloud to have a proper max_allowed_packet value.