BE 2012 SP1a - end marker unreadable

Your job can only append to the tape if there is a proper end marker to tell BE where is the end of the data on the tape.  When a job is writing to the tape, no end marker is written.  Hence you would see "end marker  unreadable".  When the job ends, the end marker is written and then the tape is appendable again.  If you have a job failure, then you might have a situation where the end marker is not written and then the tape become unappendable.

BTW, you have set your OPP to infinite.  This means that your tapes cannot be overwritten forever.  When you have exhausted your scratch tapes, you would need overwritable tapes for your jobs to succeed.  Are you aware of this?  You should change your OPP to a reasonable period.  Any BE job will append to the first tape and would require overwritable tapes when it needs a second or subsequent tape.