Good morning Sarah,
Short answer: No, Backup Exec 11.d is fully able to read and restore 10.d backup media.
Long answer: The error reported, from my reseach, appears to have three primary causes. Firstly, if the tapes were used by Arcserve for backup media, this error can occur due to differences in how they write header information to the tape. Second, is if the media has simply become damaged or corrupt. If you have found this error on multiple pieces of media, this is unlikely. Third, is to see if there is a communication issue between the system and the tape hardware, such as a SCSI contoller problem. If so, the system may just be sending or receiving incorrect information from the tape hardware.
Regards,
Greg