You have to add an ispconfig_version file to the theme. A lot of the other template files have changed as well, but the error that you are seeing is due to you not having that file. you need to place the version of ISPConfig that your theme is compatible with.
That is an example of the file. Very simple.
18.104.22.168 - Checks for a different file name though. You can see both files in my theme.