From Kolab Wiki
The Kolab Server has a web administration interface - the Kolab webadmin - to configure the server. The webadmin is available under your Kolab hostname in a web browswer: https://kolab.example.com/admin.
For the first login you can use the default administrator manager. The password was provided by yourself during the installation process. If you forgot the manager password or if you want to change it, look at the manager article. For a regular, administrative work you should create "normal" administrator accounts.
Users can login with their username (it is their UID or their primary email address) and their defined password.