Ubuntu 16.04.4 LTS, Linux kyoko 4.4.0-127-generic #153-Ubuntu SMP Sat May 19 10:58:46 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux
I have the same problem.
In my case it is also the Clipboard manager Diodon - as Wayne Stark (w-stark) wrote on 2016-05-08
Workaround: Right mouse click icon "paper clip" (Diodon), select quit and log out will be successful again.
(By the way: Diodon Preferences, if selected, will not appear as the top most window)
Ubuntu 16.04.4 LTS,
Linux kyoko 4.4.0-127-generic #153-Ubuntu SMP Sat May 19 10:58:46 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux
I have the same problem.
In my case it is also the Clipboard manager Diodon - as Wayne Stark (w-stark) wrote on 2016-05-08
Workaround: Right mouse click icon "paper clip" (Diodon), select quit and log out will be successful again.
(By the way: Diodon Preferences, if selected, will not appear as the top most window)