It was one of the indigo/purple/whatevah strings, too. I found I could pop the whole circuit board out of the control box, and nothing seems to be loose or broken that I can see, so...?
So one of the LED must have died, and it being non-replaceable lights in series, the whole thing can be thrown out, unless you have a voltmeter and a electric tape. Then you could identify the bad one, and take it out of the circuit.
Yeah, the voltmeter went the way of 98% of my stuff, and it doesn't seem worth it. Seeking funding [/non-serious] for new light strings - my tree needs more red/amber as well as the replacement string.
https://noveltylights.com