I'am using Magento Community 184.108.40.206. (Multi Store, 3 Websites with each one Store and one Storeview)
I have following Problem:
For one website within the Multistore I check if the customer have a valid vat id. For this reason I have enabled "automatic assignment to customer group" within the customer configuration. Under "Create New Account options" I have the possibilty to change the Default Group. The default value there is the "General" customer group. I have changed it to a new customer group (on website level) which I have set up before. The new customer group is assigned to a customer tax class.
if a customer register the first time - no tax will be applied. If I use the same customer for a second order everything works fine.