Can you check you are using the named control of Field1 and not the item property?
Your exact formula worked for me when i used the Named Control.
Let me know if you get any more problems
I figured this one out - because Field 1 was a choice field, I could not use the is Null or Empty run time function. A choice field is never empty. When a choice field is blank it is equal to **Select Value**. Here is the validation rule that worked:
Or using my fields: