...with error like:
Waiting for reader process to fill input buffer ...
Errno: 5 (Input/output error), reserve track
scsi sendcmd: no error
CDB: 53 00 00 00 00 00 22 7B BC 00
status: 0x2 (CHECK CONDITION) .

the medium is blank, so k3b refuses to re-blank it for you.
solution that may work is to manually force blanking the medium with:

dvd+rw-format /dev/sr0 -force -blank