I know I shouldn't answer in this way but my thread post I edited yesterday got edited by an admin again, so I'll keep it this way.
I found the solution. I read a lot of times and always put my <script src=...>
-code line above the PlaceHolderAdditionalPageHead
line in my page layout.
All the answers you provided me actually work. There is just a simple thing what doesn't work (for me).
I have to put the source-code line INTO the PlaceHolderAdditionalPageHead
-section, not above it. Just tried it out and my test script said: jQuery library is loaded
:)
This has made my day! Thanks for your suggestions. Have a nice one!
Stefan