OME - scheduled discovery not started because previous discovery still in progress

I see the following message in OME Application Logs

A scheduled discovery of all devices on a discovery range was not started because the previous discovery of that range is still in progress. If this condition continues, it is recommended that you increase the discovery cycle interval time. Scheduled time: 20170915T180002. The following managed devices are still undergoing discovery: 192.1.1.50 

in Discovery and Inventory Portal, there is no Running Task, in the Task Status view.
(all Tasks have the status Completed)

It seems that some task is stuck, but not displaying correctly?
Is there any way to troubleshoot what is going on? 
 

Grz,
Geert