Unwrap torso part creates a messed up UV map I extracted a part of a body to buid some clothing from (Unity UMA).
The original body was perfectly UV'd.
Since I want to cover a part of the neck by my shirt I added one row of rectangles from the neck to the torso by removing the neck seam and extending the shoulder seam.
Once I redo the UV mapping (unwrap), the UV map is messed up. Triangles overlap and exceed the map range.

Pictures before and after uv unwrapping


I doublechecked the seams, they are without any gap.
I even added an additional, obviously not necessary seam at the neck end.
I reseted the UV before.
I removed the seams and reapply.
I removed the seams and cut along shoulder and side like reat shirts.
I removed all material from the mesh.


The result is the same, ot at least it's always a messed up UV map.

I'm quite sure I missed any simple step, an expert will shake it's head about.
All tutorials just apply the seams and get a perfect mesh immediately.
But I'm stuck and out of any idea to solve the issue.
Fiddeling around and examing a large number of related topics I found the solution by myself.

It seems the mesh is pinned (to whatever) and has to be freed before.


Select the mesh in edit mode
Go to UV mode/window and select all (A)
Unpin the mesh in UV mode (Alt P)
Redo the unwrap in UV mode (E)


Voila!