My assessment is that I overdid/strained the upper musculature of the back while on week 10/set 4 of 60s for the PE3. I had just started the PE3 after PE2 mastery, going directly into week 9 of it. I had intense muscle burning (the normal kind) which I hadn't ever felt in those muscles yet, but then tried bringing my body upright again after faltering.
The next week was very painful and stiff. Manual therapy helped a bit. When the muscle soreness left after about a week I retained a lot of the stiffness/soreness but in a more concentrated area. The therapist found some big knots in the rhomboid area, which is likely why neck side movement brings more pain than the other. All in all though I'm seeing slow but continual improvement.
After 3 weeks away from training I've started again, as I have seen in other body parts that the GB exercises are usually therapeutic. Yes, there is a bit of acute pain when I activate those muscles for the initial leg lift-off in the PE3, but I'm able to execute week 5 of that exercise quite fine. Do I keep training in this way as long as I see continual improvement in the soreness/stiffness, or is there reason for concern?