I checked whether the folders are there, forcing the OS to show me the hidden files. Everything seemed to be alright.
Thanks fro the hint! I actually tried an older, smaller SD card that worked well earlier on. Even with this card: no luck.
I second that whole-heartedly. Slater, thank you for your great help! This is really uncommon these days and highly appreciated!
One last thought was to ban Rockbox and go back to the stock OS. Alas, the manual on vortex‘s website indicates that one needs to update the firmware through the stock OS. As of now, I cannot boot into either this or Rockbox. By the way; I tried to load the DAP through the laptop and on a regular power plug. No loading symbols came on. However, it seems to have plenty of juice as it keeps the bootscreen on throughout the night and keeps on going.
So I guess I‘ll have to buy either a Fiio M9 or iBasso DX120, leaning towards the latter.
Unfortunately, after this experience with Xduoo, it is not on my list anymore. 90 EUR is still too expensive to have the unit die on you after roughly 2 year.