Following the official documentation for Carbonio CE a thing I would do different.
I am revering to section Single-Server Installation - Automatic Script-based Installation
The script does NOT add the zextras repo!
Note: Do not do step "Preliminary Task" under Manual Installation - the script does that!
Therefore I would do:
"Step 1: Repository Configuration first"
followed by:
"Step 2: Setting Hostname"
Now I would run the downloaded:
install_carbonio_ce_singleserver_ubuntu.sh script
What is the password for the Admin webpage you might ask.
So, you installed Carbonio CE successfully with the Automatic script above. The script does not set a password for the admin user "zextras"
In your documentation go down to section:
Post-Install - Access to the Web Interface - Manage Global Administrators
Use the command to set a password for user zextras@YourDomain. YourDomain is the one you used to install Carbonio CE.