Recently updated from 1.3.8 to 1.39d

This never happened in older version but my front store is small and not used often so I do not do a lot of use in Admin but I now see many times when I click on "who's Online" in the tools that the page never loads.
Other functions in tools does work.

Other times clicking who's online and that page does load without issue but fails to load most times

Cause, how to debug, fix, something from doing the update ?

Ideas you might have to solve this ?

Thanks