i took a closer look on the rig and this is what i found:
- All objects of the character were parented to the body and had no vertex groups, only body was parented to armature and had vertex groups. They were parented to the body-object, but the armature had no influence on them still.
2. There are too many separate objects. It makes the character incredibly difficult to weightpaint. Keep the model as simple as possible - or - as complicated as really necessary.
3. There were different transformations on the model and the armature.
Im not 100% sure, but this may have caused the shoulder bug. I assume the shoulders had a transformation related to the world origin - and the armature had a transformation related to the world origin too.
When you parent the object to the armature with weights, the transformations get mixed up + there is additional influence from the rig.
Anyone please correct me, if I'm wrong.
But to avoid such errors, make sure the armature ist centered and the model as well. Also make sure you apply all transformations before parenting.
4. Loops of armor is not matching to body at some parts. That makes trouble in weightpainting. Extrude armor from body, if you can.
5. trouble with automatic weightpainting
-->arms too close to head -> arm bones deform head
--> model character in t or a pose to avoid that
- You would have to parent "Ellenbogen" bones to root bone, not arms. Otherwise you get depedency cycle between those bones and that makes arm unusable.
Here is your file back: https://we.tl/t-IP1Wo7iwz9
I hope my answer was helpful and let me know if it helped.