The symptom is the udm_inventory queue backs up with messages.
This may happen if some probes are sending a flood of messages and this will show how to determine
Release : 8.51 and 9.x
Component : udm_manager
Get a list of the udm_manager running statistics which will show the details for the messages going into the queue.
Go to Admin Console > primary hub > probes > udm_manager > view probe utility > getRunningStatistics > click the green arrow. Then click the export icon, the box with the arrow to save the output to a file > enter a file name under outputFilename
The file location is at the primary hub under: