Suspected regression from https://gerrit.wikimedia.org/r/c/427351/, Source sections are not send for saving from client side.
A restored translation looks like this:
Causes the js error
TypeError: Cannot read property 'content' of null at Object.mw.cx.dm.Translation.static.getSavedSection
The JS error was fixed in https://gerrit.wikimedia.org/r/c/429167/ but the cause for this skipped source section remains unsolved