Hello,
Sorry to hear you are having issues. But I can open Calendar by clicking on the clock/date area on the taskbar. Refer my short video below.

Please try, uninstall Start11, reboot immediately (important), download latest version available in your account product download page: https://store.stardock.com/myaccount/products . Install it, reboot immediately again. Retest it and report back.
Thank you.
Basj,
Stardock Community Assistant.