It is possible for users to log into the UMC. They will have access to exactly one module, so it will open automatically. They can change their password then. The confusing part is, that after they did that, they will automatically be presented with the same module again, because there is no other.
You can install the Self-Service App. It will provide the possibility for your users to change their password in case they forgot it. They can use email, text message or whatever you come up with to send them a token: univention.com/2015/12/self ... n-ucs-4-1/