This is usually caused by either a bad door switch (the bottom one in this case), a loose door switch mount, or a problem on the control panel.
Even though a door switch clicks, it may still be bad inside. Its continuity must be checked.
Door switch or mount trouble can be caused by slamming the door or by opening the door while it's cooking without hitting the Stop pad first.
There are plastic mounts inside the microwave which hold the door switches and onto which the door hooks latch when you close the door.
The screws on these mounts may be loose. If they get too loose, the switches will not be activated properly.
Sometimes it's a broken tab on the switch holder, allowing the switch to rotate just out of position. Other times, it's a broken tab on the switch holder, allowing the switch to rotate just out of position. This tab can be hard to see, since it is under the bottom edge of the switch.
If you or someone you know decide to look into it, we have critical information on safety, disassembly and door switches at our site, and our link is at our listing here on FixYa: http://tinyurl.com/yzjozk
We have the service manual for the R-1500 series and we have uploaded it to our site here: http://www.microwavedisplay.com/r1500_r1501_r1505_r1506_sm.pdf
If you think the problem is on the control panel, we repair these nationwide by mail for $39.95 postpaid in most cases.We're happy to help and we appreciate your thoughtful rating of our answer.