Update:
After hours of reading in the forums here and trying almost everything I found a solution.
I put the device in recovery mode (slide the lock to orange on the top + hold the rec-button + plug it into the computer)
https://kb.sandisk.com/app/answers/detail/a_id/197/~/connecting-sansa-players-in-recovery-mode
Then went to “my computer”. There it showd the 16mb partition (NEVER EVER FORMAT THIS LIKE YOU WOULD WITH OTHER DEVICES, that’s what I read). I double clicked on it. it had one TXT-file in it and I was told to create a new folder, which I should name “sansa.fmt” (without quotation marks). This is triggering a forced formatting or so.
Afterwards I unplugged the Player and it told me “Formatting . . .”
And now it is all brand new. I guess I’m gonna use it another 10 years 
I hope this might help anybody else in the future even though the device is about 10 years old.
Special Thanks to Jamieson (http://forums.sandisk.com/t5/user/viewprofilepage/user-id/38463)
