Since signs got their graphic rework, I can't place them on walls anymore. On the outer side that is. I always get this error message:
Failed Check: IsInArea (PrefabPreProcess - Client/sign.huge.wood/sockets/free/inside)
Does that mean, we cannot place signs outside anymore? Like I said - I only can't place them on the outer side of a wall. On the inside one it works fine. And if that's the case: Can I reactivate outside placing again?

Huge wooden signs are notoriously difficult to position correctly, and will not place at all in certain wall configurations. This is normal.

Well, I tried to put the "new" huge sign on the exact same wall configuration as the "old" one - where it worked without any problems. It seems to me that the new model has some issues to it

You may be right, but huge signs have always been tricky one way or another, I don't think you will find a "fix" for this.