verbs - An exception are or is?

This is forbidden. An exception is/are situations where you...

The word exception is singular but the word situations that follows is plural. Also I'm not sure what is the subject and what is the object in this sentence but I think subject vs object may affect the answer to my question.

Edit: To clarify, I'd like you to let me know if either is version is correct, or are version is correct, or both, or none.

Answer

What you're looking at is the principle of proximity. From the prescriptivist point of view: It's the subject that dictates subject-verb agreement, but there are times when subject-verb agreement isn't so easy and clear. A strict prescriptivist grammarian would tell you that only the following sentence is correct:

An exception is situations where you...

but in real life, that is not the case. Both of your answers are fine, and this is why:

Merriam-Webster's dictionary of English usage. (1994). Springfield,
MA: Merriam-Webster.