If it works fine with an and operator then I’d imagine that startDate < $regDate returns false, with an && this would abort evaluation immediately but with an or it would mean that startDate > endDate is evaluated and so presumably it is this bit that is throwing the NPE.  I’d imagine that the most likely reason for this would be that endDate is null.

Try changing it too

Segment ( startDate < $regDate || (endDate != null && startDate > endDate) )

And see whether you loose the NPE.

Thomas

rule 

       when

              Membership ( $regDate : regDate)

              Segment ( startDate < $regDate || startDate > endDate )

        then

            System.err.print(“Invalid start date”);

end