How exciting. I just looked at my Pro S and the back didn't release. Only in portrait mode though.

Anyhow, the problem has a simple cure.
Remove the back and revolving back adapter from the camera and trip the camera.
Here's what happens:
There are two flat pieces that protrude from the top corners of the body. When the shutter is released, they should pop out
and immediately return to their rest position. This is so fast it's not really visible unless you have a reference.
I put my thumb nail next to it and was able to verify the extension/retraction. One side(top right) was a bit slower so
I put a miniscule drop of Naptha down each side of it. I did both sides since I was there.
On the revolving back adapter itself there are two pins that are pushed by these fingers. One at each to corner. One looks like a
screw head, the other is plain. Same thing though, a tiny drop of Naptha on each one.
That should clear it up. But. there is the chance that the linkage in the back is sticky from old lube too. It's another simple one.
Remove the cover plate, the one facing the camera and all the linkage is visible. Nothing's going to pop out but don't turn it over.
As I remember(?) there's at least one lever just set into place Again, a small drop of Naptha on each pivot should clear it.

clear it up.
a tiny drop of Naptha to each one