Here is the uncommon SENS problem on my Win 7 computer:
The message of "Failed to connect to a windows service (could not connect to System Event Notification Service)" first appeared after installing Windows Update KB4539601 and the necessary computer restart. Slow log-in/out, the taskbar went to the grey classic theme instead of the Aero theme, audio service took time to load...
I tried to solve the problem by resetting "SENS" & "COM+Event System" in services.msc, and resetting Winsock Catalog with cmd. They worked at the first time of restarting the computer but the problem happened again after starting the computer the other time.
I went through some posts in various communities and tried to uninstall the mentioned Windows Update as suggested in this post: https://answers.microsoft.com/en-us/windows/forum/all/system-event-notification-service/03db6f99-e246-4925-88c8-2a6620c33eea?page=2
The uninstallation and computer restart took hours to complete. The Windows appeared to be normal for a time but the SENS problem re-appears every time I restart the computer. The suggested uninstallation did not work for my case. Also, resetting services & Winsock Catalog no longer work...
*** Perhaps a useful finding: the SENS problem would be solved one-off if I manually log out the user account and log in again (coming with a minor problem on delayed shut-down of explorer.exe "playing log-off sound effect" when logging out/shutting down). However, the SENS problem happens again whenever I boot up / restart the computer.
Any idea on the solution?