I've had this problem this morning (not for the first time).
I've already burned a dvd with some tv episodes (DVD-R).
I then went to burn the 2nd series but kept getting the error message.
I tried lots of fresh disks and reset the computer. It was when I was checking the length of the file names (just in case) I realised that one of the files was an .avi and the rest were mp4.
I burned the disk without the .avi file and lo and behold it's working.
Maybe it doesn't like a mix of extensions .....