If a form of the be verb is used and there is a noun both before and after that verb, the verb must agree with the subject. This holds true even if the predicate noun (the noun coming after the verb) is different in number.
The cause of his health problem was his bad eating habits.
His bad eating habits were the cause of his health problem.
(From Write for Business, page 324, and Proofreader's Guide PDF, page 74)
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