I am new to ripping. ogmrip and dvd:rip both worked fine jsut once using H264. They then refused to do any more. dvd:rip gave the same mesage as oracle_the_blind reported. ogmrip worked again after a reboot.
I am now trying handbrake as recommended. Done one successful rip and the second has started ok so keeping my fingers crossed. It seems more likely it is an underlying fault with Ubuntu or LInux..