Which causes Claudio's problem in Much Ado about Nothing?

If your question is "Who causes Claudio's problem" the answer is Don John who delights in torturing him for some reason. If your question is "What causes Claudio's problem" the answer is his lack of confidence in Hero and his friends. This is most clearly shown when Don Pedro negotiates the marriage between Hero and Claudio, and Claudio is easily persuaded by Don John into believing that Hero is bound to be unfaithful to him with Don Pedro. This is ridiculous but Claudio believes it because his insecurity makes him prone to jealousy.

The cause for Claudio's problem in "Much Ado About Nothing" is that he cannot be honest. In every situation, he has a need to be deceptive.