There is a HFix-21695 in Pega 7.1.6 release which deals with the similar error.
The root cause of HFix-21695:
"During creation of work object via agent, The calendar which is added at work object level by using pyCalendar is considered as business calendar, and the same calendar is being used for goal/deadline time calculations, But the calendar name isn't passing properly (passing blank) to the new SLA structure when SLA instance is configured with pyEscalations and business days. As if the calendar value is blank then @AddTime utility function takes the "Default" calendar instance which has been configured with two different time zones in system. Hence causing below exception.
- Multiple timezones not supported for one calendar
- The business calendar object could not be found."
Please double check your issue is caused by the same. If yes, you may need to port HFix-21695 to Pega 7.1.7.