first. system is stable. no memory problems. but: my system is having 8 gb of DC chips. running vmware images and never had problems.
second. notepad++ has released a new version 5.5.1. i installed it over my old one (<5.5) without having any reasons (keeping software uptodate) and was wondering why "open with notepad++" was missing in the file explorer context menu. so, in a boring moment i tried the "servant behaviour", and here: no problem anymore.
so, i dont know the reason, but it works for me now. i will come up with an update if the behaviour happens again (f.e. after installing another notepad++ update). it seems the origin idea of a buggy addin was right. mea culpa

@selfman: but thanks for trieing finding a solution/reason for my problem.