Office365 Calendar API - Create fails returning : "ErrorInvalidIdMalformed : Id is malformed".

Create calendar fails - trying to create a calendar under a calendar group. 

Even though, am able to manually create a calendar under the CalendarGroup by logging into the account using a browser, am not able to do the same via API. 

When I tried hitting the API, got the following error back : "ErrorInvalidIdMalformed : Id is malformed". 

It used to work fine until couple of weeks back. 

Initial investigation looks like, it mostly concerned with predefined/default calendars that come up with the office365 account : Ex: My Calendars, People Calendars, Other Calendars. Using API, am not able to POST Create a calendar with the above error.

Also, some of the Calendars that are listed in the account - am not able to delete them. UI shows "Can't delete calendar - Error Code 6". That is all the information am getting back.