Thanks for your tips, now I have a working Debian Buster on my SheevaPlug! https://tadeubento.com/2018/sheevaplug-2018-unbrick/ and OpenOCD helped me out starting the original u-boot and then installing the u-boot version of Debian Stretch as Michael suggested and updated in his docs.
I even got this working under macOS, but I had to edit /share/openocd/scripts/interface/ftdi/sheevaplug.cfg: [...] interface ftdi # ftdi_device_desc "SheevaPlug JTAGKey FT2232D B" # ftdi_vid_pid 0x9e88 0x9e8f ftdi_vid_pid 0x0403 0x6010 # ftdi_channel 1 ftdi_channel 0 […] Thanks again for your qualified and fast responses! Florian