Can you check the following in the SNC PO for which you are creating the ASN:
SNC PO > Item Details > General Tab > Confirmation by ASN.
The value should be either "Mandatory" or "Optional". If its not then also you may get the above mentioned error.
I have not made any changes to the Validation Profile - P001. Just the std SAP Validation checks are in place and its working fine for me.
Can you do the following:
1. Check in ST22 if you get any dumps after publishing the ASN
2. Also check Logs- SLG1 for any error message.