Mister (EUSSR) on 11/04/2022:

This was not sustainable, so on the second or third night, I was about to break the fast. But first I tried drinking various herbal teas with liberal amounts of lemon juice (from concentrate, with sodium sulfite preservative). The cramps disappeared quickly and continued drinking of lemon juice allowed me to continue the fast for a total of 10 days.

The remedy must have been the lemon juice, because the herbal teas were very watery by themselves. I don't know why fasting caused such terrible muscle cramps, but I bet it's either the citric acid or vitamin C that completely relieves the symptoms.