Hello Gianluca,
Can you please check the ovn north db log.
This is placed in /var/log/openvswitch/ovsdb-server-nb.log
Please check if the logs has any new entries when you try to connect and when you issue the 'ovn-nbctl set-connection ptcp:6641' command.
If the connection attempt is getting through, pvs db should print an error to the log.
Please also try restarting the ovn-northd service.
Do the ovn-controllers connect to the south-db?
You can verify this by looking at /var/log/openvswitch/ovn-controller.log on the ovn-controller host (please look for entries saying "... <north ip>:6642 connected")
Marcin