Pain when doing Manna PE1 iM

When attempting to do the MN PE1 iM with the weight in my right hand I'm experiencing soreness/dull pain on my right posterior deltoid.  The following workout I tried it without weights, but my shoulder started to feel sore on just the second set even though I don't feel like I'm pushing myself too hard.  Additionally, I'm feeling some discomfort just below the inferior angle of my left scapula (maybe part of my lat?) towards the bottom of the rep (with the weight in my right hand still).  When doing the mobility on the left side I don't experience either of these issues.  This has been going on for a couple weeks and I'm planning on temporarily ceasing this mobility work since I think I need some sort of prep work before I can return to it.  Are there any stretches or exercises that might help with either of these issues?

Can you post a form check of yourself doing the iM on both sides, this may help us suss out what going on more effectively. 

When arms are moved overhead in flexion and the place with pain is near the rear delt usually it is the rotator cuff muscle complex , especially teres minor. you can fix with  a normal stretch of the rotator cuff.

I had some problems doing this IM, and I am still not 100%.

I found doing the exercise slowly with a light weight helped.  BUT...  what allowed me to get past some pain and mobility issues was using my wife to help.   The further I went with this stretch, the more my hand would drift outward.  I had my wife use a bit of pressure to help keep my hand tracked over my shoulder.  It hurt, I had some snaps crackles and pops, but it worked wonders.  I wish I took a before and after video because the change in my ROM was insane.

Within 2 weeks my mobility change was dramatic.  I think it came from compensating due to an injury over 10 years ago.  My shoulder wasn't used to that ROM and I am sure I have some scar tissue in there.  The assisted stretched help me put my shoulder into a position it hasn't been into in a long time.  Now I can get there WITHOUT assistance. 

I make sure I go slow and focus on form form form.

It's the hand drifting out that is the most common culprit in this. Maybe you can loan the OP your wife?

Thanks for the replies.  I'll plan on posting a video next week--I'm going to wait until I'm pain free before trying the mobility again.  I do have trouble keeping my arms vertical (my left is actually worse in this respect).  My plan was to try to get my arm closer to vertical each time, but it sounds like I should definitely keep my arm vertical and possibly cut the rest of the motion short if I can't continue to keep my arm vertical.  Is this correct?  Also, I'll see if I can find someone to help keep my arm vertical.

15 hours ago, Alessandro Mainente said:

When arms are moved overhead in flexion and the place with pain is near the rear delt usually it is the rotator cuff muscle complex , especially teres minor. you can fix with  a normal stretch of the rotator cuff.

Originally I thought it was my rotator cuff, but after palpating the area I decided it was most likely my deltoid, but I'm no expert...