Great explanation, thanks a lot!
Now that you speak of it, I seem to remember I have already used the soften/harden command... anyways now that I now that nothings wrong with the geometry itself (vertices/edges/faces-wise) my heart can be at ease again
I had a problem with double geometry too using the convert commands (I think this was the cause, not sure), still don't know why that was either.

anyway thanks a bunch, that helped a lot