When I try to add any iCal URL via Settings → Calendar → Calendars → Group Calendars → Manage → Subscribed by Group, Zoho pops up an ‘Internal Server Error’ and nothing is added in the ‘Subscribed by Group’ list. 

But - the feed then appears under my personal ‘Subscribed’ calendars instead.

I have reproduced this with multiple public ICS URLs (Timetastic and Google UK holidays).