To access my Zoho calendar, I go to mail.zoho.com in a browser (which brings up mail) and then click on Calendar to display the calendar. This worked through yesterday (Wednesday evening, November 8, 2017, US EST).
This morning (Thursday, November 9), when I click on Calendar, the calendar is a blank. the icons are still there on the left, and the other icons (Mail, Tasks, Contacts, etc.) work as expected. But the calendar screen is blank. This is using the Qupzilla browser on Linux (Arch on x86_64). Using Firefox (same computer), when I click on the Calendar icon, whatever was there remains (e.g., if I had Tasks displayed when I clicked on Calendar, Tasks continue to be displayed).
I patched and rebooted my machine this morning, but made no changes to any configuration. Also, everything else is working, including Zoho Mail as noted above.