I was unable to find a hint where I have to place the scripts for the "mantra -P pythonscript.py" filter functionality. It works fine with an absolute path, but I'd prefer to place it in a more common place in the PYTHONPATH. So where exactly does mantra search for the script?

I don't think it will work without an absolute path as it's essentially read/executed like a script and not imported in the standard Python way. Depending on how you're formulating your mantra command you could try putting it somewhere on the HOUDINI_PATH and then finding the script as a relative path via hou.findFile:

Ah, okay. Thanks thanks a lot. This will help... and it seems that placing the file in the HOUDINI_PATH works without the absolute path.

Ah, that's cool it will work just being in the HOUDINI_PATH without needing the absolute path. Can't believe I never tried that. Cheers!