I have this problem now also with DNN 5.6.0. Before I did not see it. Has somebody logged it into Gemini already?

This problem is still here in v5,5 
I believe we have a typical read/write lock  issue, I  have  cleared schedule history and event log.

Question Should I truncate the transaction log. (Normally a very Bad thing to do.)

Starts with OutofRange ERROR   down in 
  PropertyName>InnerException</PropertyName>
System.IndexOutOfRangeException: Collection index must be in the range 1 to the size of the collection. at Microsoft.VisualBasic.Collection.IndexCheck(Int32 Index) at Microsoft.VisualBasic.Collection.get_Item(Int32 Index) at DotNetNuke.Services.Scheduling.DNNScheduling.Scheduler.CoreScheduler.ScheduleQueueContains(ScheduleItem objScheduleItem) at DotNetNuke.Services.Scheduling.DNNScheduling.Scheduler.CoreScheduler.AddToScheduleQueue(ScheduleHistoryItem objScheduleHistoryItem) at DotNetNuke.Services.Scheduling.DNNScheduling.Scheduler.CoreScheduler.WorkCompleted(SchedulerClient& objSchedulerClient)</PropertyValue>

Then thelocak  timeOuts begin. a couple of reads  then a write

System.Threading.ReaderWriterLock.AcquireReaderLockInternal
System.Threading.ReaderWriterLock.AcquireReaderLockInternal