When I try to run a second instance of pirut, I get an error message:
  "Another application is running which is accessing software information."
When I click OK, pirut ends.

But then, I try again to run the second instance, without ending the first one.
And voila, it starts up without any complaint.
(And by the same mechanism, I can run as many instances of pirut in parallel as
I want.)

Apparently, pirut clears the lock on exit, even the log was granted to another
instance.