On the UC admin page select Message Store > Mailbox Stores > and select the new mailstore you created.
On the edit Mailbox store page you'll see a check box that is labeled as 'Mounted' and it should have a check box if it is.
You are correct that a demo license will allow you to add 10 users and at least a few templates (I'm not sure if that has a limit or not but you can add them). This type of an error would lead me to believe that you have some type of a database issue. I've seen this problem where you had users that you couldn't delete however getting this when you're adding someone is interesting.
How exactly did you add the user (LDAP or new stand alone user)?
Did you have a server that you upgraded to get to this version or is it a new install?
When adding a user with a mailbox you'll need an alias, extension, and you'll have to select a template. Have you tried with different templates?
Are you using the same alias/extension for these users?
Where you ever able to install any users with mailboxes?
I would say that if the fact that you can't add a template may be the cause of this. If you are using a valid alias and an extension then the only required piece would be the template. If this is a new install you may want to reinstall the server just to find out if that install didn't have a problem.
Are you getting this error “Installer User Interface Mode Not Supported. The installer cannot run in this UI mode. To specify the interface mode, use the -i command-line option, followed by the UI mode identifier. The value UI mode identifiers...
The below trick might come handy when you have to add a new node to a cluster but you don't have or is unsure of the security password for the publisher. This procedure has been around for ages.
1) Login into the CLI of the Publisher.