For the first time the credentials are root and no password as stated in the documentation.
The wizard does ask you for new root password and you can not skip it. So the password for root was configured.
Later in the wizard you can define user under what the application will run. You can use root but you can define new user.
The login to GUI is the one the application is running.
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another reason for not being able to login may be: the XMC is configured to use AD/LDAP/Radius as backend authentication service.
Regards
ZdenÄk Pala