Can anyone provide me a pointer as to the reason for these log entries and how to clear this one up

Active Sync is currently behaving much better than all previous versions

 

[2017.06.06] 09:04:20 System.InvalidOperationException: Collection was modified; enumeration operation may not execute.
[2017.06.06]    at System.Collections.Generic.Dictionary`2.Enumerator.MoveNext()
[2017.06.06]    at System.Linq.Enumerable.Any[TSource](IEnumerable`1 source, Func`2 predicate)
[2017.06.06]    at MailService.Protocols.ActiveSync.Helpers.SyncCalendar.#z0k(KeyValuePair`2 #Ngb)
[2017.06.06]    at System.Linq.Enumerable.WhereEnumerableIterator`1.MoveNext()
[2017.06.06]    at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
[2017.06.06]    at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
[2017.06.06]    at MailService.Protocols.ActiveSync.Helpers.SyncCalendar.SaveItems()
[2017.06.06]    at MailService.Protocols.ActiveSync.Helpers.SyncCalendar.FilterAndGetSoftDeletedItems(Dictionary`2& smSyncItems, Dictionary`2 mappedItems)
[2017.06.06]    at MailService.Protocols.ActiveSync.Helpers.SyncBase.HasChanged()
[2017.06.06]    at MailService.Protocols.ActiveSync.Commands.Ping.Execute()
[2017.06.06]    at MailService.Protocols.ActiveSync.VersionSpecific.VersionHandlerBase.ProcessCommand()
[2017.06.06]    at MailService.Protocols.ActiveSync.ActiveSyncProcessor.ProcessCommand(EasCommandRequest request)