"Error cracking CSS key" error while running dvdbackup on video DVD disk

[Yuri]

I know that similar question was already asked in numerous groups/mail-lists but
I couldn't find the definitive answer.

Sometimes when I run dvdbackup (port sysutils/dvdbackup) I get the following error:
libdvdread: Error cracking CSS key for /VIDEO_TS/VTS_01_0.VOB (0x00003ae0)
and many other ones similarly looking.

After some debugging I found that error originates in libdvdcss, in file 'ioctl.c':
...
    struct dvd_authinfo auth_info;
    
    memset( &auth_info, 0, sizeof( auth_info ) );
    auth_info.format = DVD_REPORT_TITLE_KEY;
    auth_info.agid = *pi_agid;
    auth_info.lba = i_pos;
    
    i_ret = ioctl( i_fd, DVDIOCREPORTKEY, &auth_info );
...
here lba = 15072 and agid=0 (in one of the cases).

Simultaneously dmesg reports another error line:
acd0: FAILURE - REPORT_KEY ILLEGAL REQUEST asc=0x6f ascq=0x04
And stderr message is being printed:
libdvdread: Error cracking CSS key for /VIDEO_TS/VTS_01_0.VOB (0x00003ae0)

So is this likely a bug in libdvdcss, libdvdread or ATAPI driver?

Yuri