Symptoms

The following error is being recorded to the event log around 10 times per second:

24148 The User Manager 127.0.0.1 returned an empty queue.
 

Diagnoses

Normally this error is logged when there is an empty role or group (this includes role which contains empty group). It can be a little bit difficult sometime to trace this error to specific empty role or group but generally this can be quite easily done as you supposed to know roles as well as subset of AD DS groups used by K2.

In case you are sure that you do not have any empty roles or groups then it can be caused by deleted roles which formerly were used by your processes. When a process uses a K2 role in a Destination Rule, it becomes a Destination Queue. The K2 role result becomes the users and/or groups which are linked to the Destination Queue. There is also a ProcInstDestQueue table which holds a relationship to a Process instance and DestQueue. When K2 processes are stopped or in error status, the K2Server ProcessDestQueue table will still process these queues. When a process is Finished, the ProcInstDestQueue table relationship will be removed, but the DestQueue and DestQueueUser tables will still contain these entries and will keep on updating by design.
 

Resolution

There is no supported way to clean up these tables using K2 GUI. Safest way to deal with a role which formerly was used by process but no longer used/exist at the moment is to do the following: If this role was marked as Dynamic K2 will keep attempts to resolve non-existing role(s) every 10 seconds generating 24148 errors in K2 host server log. If you set Dynamic flag to 0 it will still keep updating attempts according to Interval setting (default is 28800 = 8 hours) - which is already far better/more acceptable than log flooded with numerous (depending on number of deleted roles) 24148 error every 10 seconds.

Using this approach you can tune down frequency of 24148 error at the moment but there is no supported way of getting rid of non-existing and no longer used in any active process roles but there is existing feature request to add such functionality.