After upgrade to 18.04 - PC does not shutdown on the first attempt

Dear,

I upgraded my 2 laptops from 16.04 to 18.04.1 LTS. Each time I try to shut it down by pressing the Button in the Mate-bar I get a message: A application is still running - Application unknown - no response.

I tried to figure out which application this is by using top, but failed .
Does anyone have similar problems?
how do I find out which application causes this problem?

thank you for your help
christian