How does reusing IV for OFB mode breaks the scheme
OFB converts a block cipher to a stream cipher (As does CTR mode). The plain text or any derivitive from it never enters the block cipher. The key stream is unrelated to what is being encrypted. The final ciphertext is just a xor of the plain text with the key stream and the key stream depends only on secret key and IV. reuse both and you get the same key stream. It is the same as reusing a one time pad. It's bad even without knowing a cipher text plain text pair. If you do, it becomes trivial to decrypt any message reusing the same key+IV.