@Mikael *********
they probably will accept it once you can show a Thai bank account where you actually transfer the money into. It is normal that they want to see bank records even if your embassy still issues the income affidavit.
My father, from Denmark, uses the affidavit too, and some years they want to see his bank statements from Thailand, to show money incoming.
In your case, if you didn't even have a Thai bank account yet, that's probably why.