Thanks Roms. I downloaded Tandberg's Tools and did an erase with that (short erase) and it looks like works. Just started the job and no error.
Long erase in Veeam did not work.
My conclusion is that there must be something on the tape that BackupExec has written that Veeam are not able to erase.
Then i just have to sit down and do a erase on all tapes again