After some hours of debugging, I found out that a function, deep down, which does:
let thisDay = DateIn.getDate();
with "undefined" as a value for DateIn, simply stops executing any code (normal), but does not display any error, not in Preview, not in Publish/console, nothing. It just halts.
Didn´t we use to have some kind of an "invalid date" error or something, with a line number?