Exact same problem, blugin version 3.0.8.
All works fine with first calendar, but but when I add other, I get this error in Grid view, list view works normally. First one works still normally, problem is only second calendar.
Warning: array_combine() [function.array-combine]: Both parameters should have at least 1 element in /home/customers/s2cvk/public_html/wordpress/wp-content/plugins/google-calendar-events/includes/calendars/views/default-calendar-grid.php on line 318
Warning: array_intersect_key() [function.array-intersect-key]: Argument #2 is not an array in /home/customers/s2cvk/public_html/wordpress/wp-content/plugins/google-calendar-events/includes/calendars/views/default-calendar-grid.php on line 318
Warning: Invalid argument supplied for foreach() in /home/customers/s2cvk/public_html/wordpress/wp-content/plugins/google-calendar-events/includes/calendars/views/default-calendar-grid.php on line 322