Could it be that upgrade from 78 to 91 causes a temporary export/import (via .ics file) of the local calendar and import failed due to Bug 1727938? Resulting in the calendar being empty after upgrade...
Could it be that upgrade from 78 to 91 causes a temporary export/import (via .ics file) of the local calendar and import failed due to Bug 1727938? Resulting in the calendar being empty after upgrade...