I'm unable to select a calendar in tools/holidays if there is only one branch in Koha, the Javascript doesn't load the default branch.
The problem is only here with a user who does not have a default branch (as the user Koha).
It is known that many parts of Koha will not work if you are logged in as the koha user. You should always create a new administrator patron before beginning to configure Koha in the staff client.