No problem! That has surely been an erroneous procedure of mine.
How I came across the tweaks plugin (that seems to really be great, if it works), was a very different problem, that I until now not found in any forum.
When I start the KT, there appear at the booting, for a short time, some pre-installed books (such as Umberto Eco, but also Kobo Read on an so). Even having archieved for example the Umberto Eco (that is possible under Fw. 2.4.0), it still appears. After some seconds the screen clears up, and there appear my "own" books that I'm reading. My reasoning was that the booting could become faster, if the system won't access these "Kobo files" first. I had hoped that with the plugin this could be managed.
Deleting the files has not been possible, neither under Calibre ("virtual file"). Someone knows the problem?