Zoho calendar not syncing with Thunderbird

Zoho calendar not syncing with Thunderbird

Last week I realized that I could setup my Thunderbird email client/calendar to sync with my zoho calendar.  I went through the setup steps but the two calendars have not been able to synchronize. I have tried to sync a new calendar and I am told that I am already linked to the zoho calendar.  I am also not able to delete the calendar. What can I do to fix this issue? 

Bryan