I have the latest version of ISPConfig (3.0.5.4p8) with ispc-clean theme. When I login as admin, I can successfully log on as a client. In the ISPconfig is only one reseller. Under the resellers account were created all user accounts, but reseller cannot login as his client. Error message is "You don't have the right to 'login as'!". Thank you for advice how to correct this.
Problem solved. After changing theme to default, I had to copy a few files from the original installation. Thank you.