We migrated from 2013 on-premise, to 2016 on-premise, everything went smooth with the exception of a generic user account setup for scheduling purposes. When we migrate this account mail continues to work, but the calendar sync quits working. Unfortunately The calendar is manually added to just over 100 phones. Creating a new calendar is not a reasonable option. Logs show no errors. We moved over 1000 mail objects without issue. This is the only issue. Any thoughts or suggestions?

