Hello,
just if it could help: same problem here tested with 2 LTO-drives LTO5 tandberg and LTO6 IBM
I wasn't able to read or label a new tape, with the same error:
proxmox-backup-proxy[2878]: GET /api2/json/tape/drive/LTO-01/read-label?: 400 Bad Request: [client [::ffff:127.0.0.1]:48742] read...