For test, can you please run at MD:
ls -lha /home/public/data/videos/DVD/Alexander.dvd
? to see if it reads OK. if you have few minutes, run also
md5sum /home/public/data/videos/DVD/Alexander.dvd
on both MD and Core, to see if it matches (can be a network error)
Also, do you have libdvdcss installed on M/D? If DVD image is encrypted, absence of libdvdcss can also cause this