Apparently Ritek had some quality control issues about four months or so ago. Your discs may be affected by this.
For the QScan graph you posted, notice that you have both tracking and focus issues, since both lines are exceeding the height of the dashed reference lines (about 440 for focus and 500 for tracking). Also notice that these errors exist near the BEGINNING of the disc. I too have seen similar QScan reports from my Ritek media, also near the beginning of the disc, and I have learned to just ignore them, because I always get good burns.
Since your problems occurred when you were scanning near the MIDDLE of the disc, it follows that they might be related to another issue - or possibly just a single bad disc.