Thanks, all!
It was some kind of learning experience. After using Miles’ sketch, nothing happened again. His “Hello world” in combination with the jyetech scope showed me that something was wrong with the serial world, too. I re-wrote the bootloader, applied his sketch again - and wow, is this matrix bright! 
Write the original sketch - fail.
Deleting original sketch .o files etc, then rebuild with correct platform set, write to Rainbowduino - I ger a test pattern! YAY!
I should be on my way now.
Again, thanks for your help!
Best,
Jens