The customer has upgraded his environment (in this example from 8.5 RU4 to 8.6 RU3 version). His Site Server's Deployment Task Server Handler is still on the previous version (in this example, v8.5.5710). It hasn't upgraded after two days.
ITMS 8.5. 8.6
Upgrade policy (Deployment Site Server Components (x64) - Upgrade) wasn't enabled after upgrading to the new version.
Check the following:
191333 "error: COM object 'DeploymentSolutionAgent_Task' is not found, make sure the corresponding plug-in or task handler module is installed and registered"