So I talked to the people who are hosting today, and they had indeed done a mistake replacing the tapes. It was now completely full. I asked them to remove one (the 24 slot, VolumeTag=ARS655L3) and he did. Now, I was going to follow your procedure, and started with a status to see if everything looked ok. That gave me this result:
# mtx status
mtx: Request Sense: Long Report=yes
mtx: Request Sense: Valid Residual=no
mtx: Request Sense: Error Code=70 (Current)
mtx: Request Sense: Sense Key=Unit Attention
mtx: Request Sense: FileMark=no
mtx: Request Sense: EOM=no
mtx: Request Sense: ILI=no
mtx: Request Sense: Additional Sense Code = 28
mtx: Request Sense: Additional Sense Qualifier = 01
mtx: Request Sense: BPV=no
mtx: Request Sense: Error in CDB=no
mtx: Request Sense: SKSV=no
Mode sense (0x1A) for Page 0x1D failed
mtx: Request Sense: Long Report=yes
mtx: Request Sense: Valid Residual=no
mtx: Request Sense: Error Code=70 (Current)
mtx: Request Sense: Sense Key=Not Ready
mtx: Request Sense: FileMark=no
mtx: Request Sense: EOM=no
mtx: Request Sense: ILI=no
mtx: Request Sense: Additional Sense Code = 04
mtx: Request Sense: Additional Sense Qualifier = 00
mtx: Request Sense: BPV=no
mtx: Request Sense: Error in CDB=no
mtx: Request Sense: SKSV=no
READ ELEMENT STATUS Command Failed