Hi 1mann1wort,
It is difficult to say anything in general, as I don’t even know if the character is driven by some XPresso for example. Too many variables. With that being kept in mind:
As a general rule, if you use a Null, never ever use the Group -Short-Cut, it will destroy any animation. Use a fresh Null, and place the objects while the Null stays at zero P+R and one at scale.
If you scale the project (project-scale not scale-project) and save it, then merge it in a new scene, scale the original again, save it and merge it into the new scene as well, is there a difference to the original scene (not scaled) if you merge it as well in the new scene?
Here is a little clip with failures and successes to scale.
https://www.amazon.com/clouddrive/share/sqN8NTLvG7bl4FrymZIJc6OAcFBTZJwYov4ARr25yiD
Yes, and yes, every character is different…
Anyway, without the scene file it is hard to tell anything without going on with guessing.
Do you need an upload link or do you have a cloud service, etc.?
All the best
P.S.:
Since I don’t know what kind of rigging you have used, this might be right or wrong to suggest:
https://www.cineversity.com/vidplaytut/siggraph_2015_rewind_bret_bays_weighting_workflows_for_cinema_4d_character
@44:32 [min:sec]