I have created this rule several times now and I get the same result each time. I was moving a RM legacy rule to RM5.1 on another hub, so I am rebuilding from scratch not an import. I have been able to reproduce several times now with the simple rule I created, shown below.
I create the rule and then make three variables, one string and two date/time.
I set both date time variables to 12:00 AM with no date. String I set to "unknown"(See 2nd pic)
I then start editing the rule and after adding the IF statement and setting the StartTime variable to 12:00AM, when I then try to set the value of StopTime it hangs.(See first picture.) I never get the drop down so I can set the date or time for the Stop time variable. The rule is shown in the second picture.