Well I tried to restore the player again last weekend.
I passed all the steps from
https://www.rockbox.org/wiki/SansaAMSJTAG.html (halt device, upload firmware, resume execution) but it was unsuccessful.
It looks I encountered almost the same failures that
dongs https://www.rockbox.org/irc/log-20161231 overcame in the past.
The J-link clone works well, the CPU core ARM926EJ-S is detected.
Next he wrote about memory remapping.
if I jump to 0x0 and start executing from there, it runs, then gets stuck in a endless loop somewehre in 0x80004xxx or somesh*
I guess my next step also has to be memory remap and load Rockbox bootloader.
And I again postponed further attempts, since my knowledge of debugging is very superficial. I'm still reading a voluminous topic from the archives.