Thanks Cathal, solved it for me also.

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I'm receiving something very similar  on my 7.3.1 environment. 

This  bug  supposed to be fixed in  7.3.1 : DNN-5349 

Tons of Scheduler Errors of this sort: 

PortalID:-1

PortalName:

UserID:-1

UserName:

ActiveTabID:-1

ActiveTabName:

RawURL:

AbsoluteURL:

AbsoluteURLReferrer:

UserAgent:

DefaultDataProvider:DotNetNuke.Data.SqlDataProvider, DotNetNuke

ExceptionGUID:b9c09bdc-7a5e-4e25-8bfb-f4ddd8155b9e

InnerException:Sequence contains no matching element

FileName:

FileLineNumber:0

FileColumnNumber:0

Method:System.Linq.Enumerable.Single

StackTrace:

Message:

System.InvalidOperationException: Sequence contains no matching element

   at System.Linq.Enumerable.Single[TSource](IEnumerable`1 source, Func`2 predicate)

   at DotNetNuke.Services.Scheduling.Scheduler.CoreScheduler.GetServer(String executingServer) in d:\DEV\DNN_Platform_07.03.01_Source\DNN Platform\Library\Services\Scheduling\Scheduler.cs:line 263

   at DotNetNuke.Services.Scheduling.Scheduler.CoreScheduler.LoadQueueFromTimer() in d:\DEV\DNN_Platform_07.03.01_Source\DNN Platform\Library\Services\Scheduling\Scheduler.cs:line 687

   at DotNetNuke.Services.Scheduling.Scheduler.CoreScheduler.Start() in d:\DEV\DNN_Platform_07.03.01_Source\DNN Platform\Library\Services\Scheduling\Scheduler.cs:line 882

Source:

that's a different error from the other one - could one of you (i.e. John Cant or SCTAN J ) please do a select * from webservers query and post the results here -it will help us diagnose what could be wrong

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