I suspect that someone over (nut) tightened and stripped that particular section of threaded tube.

I'd have the threads die chased, wire brushed clean of any debris, then add (or machine a collar if necessary) a taller washer / spacer to extend the nut grip area onto the upper (good) thread section along with new nuts. You'll be good to go!

Another option MAY be to use a thinner washer / spacer (if clearance and type of current assembly components permit) to allow the nut to ride LOWER on the good threaded section.