Scheduler Issues in DNN 7.3.x

DNN > Troubleshooting
After upgrading to DNN 7.3.x, a bug in DNN can cause scheduled tasks scheduled via Host > Schedule to not run properly. Looking in the DNN event log, you may see an error message such as this one:
 
LogType = SCHEDULER_EXCEPTION

AssemblyVersion = 7.3.4
PortalID = -1
UserID = -1
ActiveTabID = -1
RawURL = /Admin/SecurityRoles/tabid/41/Default.aspx
AbsoluteURL = /Admin/SecurityRoles/tabid/41/Default.aspx
AbsoluteURLReferrer = /Admin/SecurityRoles/tabid/41/Default.aspx
UserAgent = Mozilla/5.0 (Windows NT 6.2; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/41.0.2272.118 Safari/537.36
DefaultDataProvider = DotNetNuke.Data.SqlDataProvider, DotNetNuke
ExceptionGUID = 104d0622-e897-4476-8b54-e365f938b0f3
InnerException = Sequence contains more than one matching element
FileLineNumber = 0
FileColumnNumber = 0
Method = System.Linq.Enumerable.Single
Message = System.InvalidOperationException: Sequence contains more than one matching element
   at System.Linq.Enumerable.Single[TSource](IEnumerable`1 source, Func`2 predicate)
   at DotNetNuke.Services.Scheduling.Scheduler.CoreScheduler.GetServer(String executingServer)
   at DotNetNuke.Services.Scheduling.Scheduler.CoreScheduler.LoadQueueFromEvent(EventName eventName)
   at DotNetNuke.Services.Scheduling.Scheduler.CoreScheduler.RunEventSchedule(EventName eventName)
This occurs when there is a duplicate entry in the WebServers table. To verify if this is the issue, go to Host > SQL and run the following SQL query:
SELECT ServerName FROM {databaseOwner}[{objectQualifier}WebServers]
If you see the same entry more than once, you need to run the following SQL query: 
DELETE FROM {databaseOwner}[{objectQualifier}WebServers]
Once that has been completed, hover over Tools and click Recycle Application Domain.
 
This will remove the duplicate entry, and cause all web servers that are registered with DNN to be removed and re-indexed. 

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