Hi,
Like what Arijit and Anand has said, I'm also not able to understand the exact problem. If there is no follow up action associated with the relevant UD code, then SO creation (parallel, and based on UD code) is out of general understanding.
May be you are missing some info here, or you might need to recheck the steps you are executing or settings maintained.
You must be knowing that making UD is an independent transaction which doesn't (until and unless something is not defined in background) make any other transaction.
I understand that as soon as some particular UD code is entered, system is looking to create a Sales order and due to restricted access, SO doesn't create and thus user is unable to make the UD. Therefore, there has to be some enhancement, development or whatever you say in the background !!
Or else, you are misreading the error message.
Besides, Stock posting and UD are again different as Anand already mentioned this. Did you try to make stock posting only ? And are you using notifications? Is there a probability which could trigger SO creation following notification ?
I'm confused 
ntn