Ok, it's getting exciting.

@Estel: Thanks for your post. Very likely the cause for this was something I installed recently and as it is related to the same hardware, USB mode 1.0-2 comes to my mind, but I am not 100% sure.

/usr/sbin/osso-usb-mass-storage-enable.sh /dev/mmcblk1p1 works, the SD card pops up on the computer. Same for the internal memory.
Means I was using the script in a wrong way earlier without handing over the dev.

So it is very likely, that only the popup which starts the exporting process is faulty or just is not triggered. So #3 from my last post is solved. The N900 can be used as mass storage, when triggered manually.

Any hints what makes the PopUp fly to solve #1 & #2?

Thanks a lot,
Enrico