After backing up and then restoring the backups in a new environnement, only one user is able to connect to Confluence.
User mangement is done by a Jira instance, to which all users are able to connect.
Only one admin can connect to both Jira and Confluence. Another admin, in the same groups, can not connect to Confluence (message is "Sorry, an error occurred trying to log you in. Please try again.").
Jira backup and restore was done by internal XML backup + attachements copy/paste.
Confluence backup and restore was done by internal Zip backup, which was only restored after initially reconnecting the user management with the new Jira instance.
Old environnement: Windows Server 2012 R2, PostgreSQL
New environnement: two Windows Server 2016 machines (one for Jira, one for Confluence and PostgreSQL).