Have been using xlights for 4 yrs. Have had this problem for the last ~ 2yrs. Using xlights v 2019.64.
I was cleaning up some errrors in sequences, paticularly fade in and out times exceeding duration of effect. When I backspace in fade in or fade out to enter a new value, it will not allow me to type in a new value.
I have found a workaround, create a word doc, type the value, copy, and paste into the fade in/fade out panel. This is not super critical, but a bit of an annoyance.