I get this error - please advise

Constraint

Action: ReceiveCalendar
Calendar: Calendar

SQLite.SQLiteException: Constraint
at SQLite.PreparedSqlLiteInsertCommand.ExecuteNonQuery (System.Object[] source) [0x00178] in <84b9c9e630fa45bd8ac799333976ebbf>:0
at SQLite.SQLiteConnection.Insert (System.Object obj, System.String extra, System.Type objType) [0x000ef] in <84b9c9e630fa45bd8ac799333976ebbf>:0
at SQLite.SQLiteConnection.Insert (System.Object obj) [0x00012] in <84b9c9e630fa45bd8ac799333976ebbf>:0
at XCalendarApp.SQLite.Repositories.RepositoryBase`1[T].Insert (T entity) [0x00006] in <f62bc91f4d704b99852e85142d4fa38b>:0
at XCalendarApp.SQLite.Repositories.CIExpandedRepository.Insert (XCalendarApp.SQLite.Entities.CIExpandedDataEntity entity) [0x00025] in <f62bc91f4d704b99852e85142d4fa38b>:0
at XCalendarApp.ProviderBase.StorageManager`8[TCachedCalendar,TProviderCalendarItem,TCachedAppointment,TProviderAppointmentItem,TCachedContacts,TCachedContact,TProviderContactItem,TAppointmentDbData].AddCIExpanded (XCalendarApp.SQLite.Transaction transaction, XCalendarApp.Model.Data.ICalendarItem calendarItem, XCalendarApp.Common.Timezones.TZDateTime tzStartDateTime, XCalendarApp.Common.Timezones.TZDateTime tzEndDateTime) [0x0008b] in <c20640e5815f4d379695772f2e8b118d>:0
at XCalendarApp.ProviderBase.StorageManager`8[TCachedCalendar,TProviderCalendarItem,TCachedAppointment,TProviderAppointmentItem,TCachedContacts,TCachedContact,TProviderContactItem,TAppointmentDbData].AddCIExpanded (XCalendarApp.SQLite.Transaction transaction, XCalendarApp.Model.Data.IAppointment appointment) [0x0000f] in <c20640e5815f4d379695772f2e8b118d>:0
at XCalendarApp.ProviderBase.AccountProviderBase`9[TCachedCalendar,TProviderCalendarItem,TCachedAppointment,TProviderAppointmentItem,TCachedContacts,TCachedContact,TProviderContactItem,TAppointmentDbData,TAccountOptions].UpdateExpandedDataForAppointment (XCalendarApp.SQLite.Transaction trans, TCachedAppointment appointment) [0x0009b] in <c20640e5815f4d379695772f2e8b118d>:0
at XCalendarApp.ProviderBase.AccountProviderBase`9[TCachedCalendar,TProviderCalendarItem,TCachedAppointment,TProviderAppointmentItem,TCachedContacts,TCachedContact,TProviderContactItem,TAppointmentDbData,TAccountOptions].UpdateAppointmentAndExpanded (XCalendarApp.SQLite.Transaction trans, TCachedAppointment appointment) [0x0004b] in <c20640e5815f4d379695772f2e8b118d>:0
at XCalendarApp.MicrosoftGraphProvider.MicrosoftGraphAccount.SyncReceiveCalendarFromProvider (XCalendarApp.MicrosoftGraphProvider.CachedMicrosoftCalendar cachedMicrosoftCalendar, XCalendarApp.Model.Data.SyncResults.SyncResult syncResult, XCalendarApp.Common.Timezones.UtcDateTime syncStartDateTime, XCalendarApp.Common.Timezones.UtcDateTime syncEndDateTime, System.String deltaLink) [0x004c8] in <bf12b9043e804a5ab4137d67b5520213>:0
at XCalendarApp.MicrosoftGraphProvider.MicrosoftGraphAccount.SyncReceiveCalendarFromProvider (XCalendarApp.MicrosoftGraphProvider.CachedMicrosoftCalendar cachedMicrosoftCalendar, XCalendarApp.Model.Data.SyncResults.SyncResult syncResult) [0x00142] in <bf12b9043e804a5ab4137d67b5520213>:0
at XCalendarApp.ProviderBase.AccountProviderBase`9[TCachedCalendar,TProviderCalendarItem,TCachedAppointment,TProviderAppointmentItem,TCachedContacts,TCachedContact,TProviderContactItem,TAppointmentDbData,TAccountOptions].SyncReceiveCalendar (TCachedCalendar calendar, XCalendarApp.Model.SyncReason syncReason) [0x0011f] in <c20640e5815f4d379695772f2e8b118d>:0