@LarrssonDK
But I am pretty sure now: the Update 3.20 works fine in first place. But as soon as you open the Newsletter–Subscription page in the Admin-Panel, die custom texts are being deleted/overwritten by default texts.
I am not quite sure sure this, but maybe it has something to do with the new Error-Page-Field: if you have already put in your own text here, ist fine and all the other custom content is being kept. If there is still the (english) default text in this field, all the other fields are being overwritten.