After the last update, the “notes widget” appears transparent with no borders. I’ve tried to change colors in the configuration menu but is useless, the widget remains transparent. Does anybody have a solution?
Thank you 
This widget has nothing to do with KNotes.
Anyway, the problem you are seeing is likely this:
https://bugzilla.opensuse.org/show_bug.cgi?id=958519
In short: the notes widget’s graphics have been removed in Plasma 5.5 and moved to Frameworks with 5.17.0 instead.
Unfortunately we forgot about this problem and only released the Plasma update so far (but then, this fixes a lot of other problems).
A Frameworks update to 5.19.0 has been submitted meanwhile too though (including fixes for the session management problems), so it should soon work again.
Please see the bug report for workarounds.