Problem stil persist after reinstallation of VUM as well as after reseting VUM database. Two new explorings:
1. The last completed "check notification" task usually appears in the same time - on 21:07:01 CET. It's just before vSphere Data Protection appliance finished VM's backups. Could it be the reason VUM loose connection to vCenter (or vice versa)? VUM is installed on W2008 VM.
2. I can see "com.vmware.vcIntegrity Task: Add Tag" in W2008 VM events just after I connect vCenter with vSphere Client from this VM. I don't restart VUM on W2008 before so it looks like some W2008 <--> VCSA connectivity problem could be the reason. But ping works.