Use Dell's rwrite.exe to write 100 MB to the drive, and you'll be able to do quick/long erase and label after that. I know it sounds too easy, but that's what I found from my experience. Obviously your data won't be accessible, but at least you don...
I'm having this same issue, BE12.5, just happened since the new year. Job has been running with Encryption Key, Dell LTO Ultrium-4, fully patched, and jobs for "Monday" have always worked up until now. I can't do anything to the tape, label, erase, ...