It seems, that I'm in the end I'm right - I encountered "the source" :-)
I posted already in the Nokia forum for Phone Applications - but I didn't get an answer.
I had the Youtube app installed, but then I had to put it to the Nokia Care Center - there was some charging problem, and it was tried to solve it with a deep reset. It didn't help - and later I found out, it was something completely different.
The Youtube app was installed on my memory card, and of course I removed that card before the reset.
After the reset I installed the memory card again - if found all applications, but not Youtube - I thought, it would have been deleted.
So I tried to install it again - Ovi Store didn't say anything and installed Youtube again.
When I left to the menu, I wasn't able to find it - also in the file manager I cannot find it - but in the program manager I suceed - but in the program manager is no possibility to start the program.
Even deinstalling and reinstalling didn't help.
Then I feared, I wouldn't be able to see any new loaded app, and installed something other - but it worked. I see it in the menu, in the file manager and in the program manager - everything OK.
Now I found some informations on UIDs and .pkg-files - but I cannot find all these files in my explorer, if I connect the mobile to my PC.
Can you help me, please?