Did some experimenting again. Kept the Open_Base_dir as-is. This works like a charm without difficult things like mounting.
Fixed the downloading of files also again by changing the last line in fetch.php to:
I don't know where you use the setting $config['cops_x_accel_redirect'] further but changed this one back to 'location' as this was the setting before experimenting.
Detailed view of a cover image still results in an error as described above.
Below you can find two images:
1 ) Error show when clicking on a cover thumbnail in order to see a larger version of the cover
2) Example of how fancybox messes with the book details.
Tried to include some screendumps directly from the PRS-T1, but the screendump feature only sees the background info and not the contents of the fancybox.