Answer:
The error in the sentence is highlighted below:
John (is one of those) bank presidents who (is) always worrying (about) making loans to young married couples (with no) established credit.
Step-by-step explanation:
At first, the sentence may seem flawless. However, there is a problem with the verb agreement right after "who". It turns out that this "who" does not refer to John. It actually refers to "bank presidents", which is plural. Thus, the verb should also be in its plural form. Since "is" is singular, we have an error. The correct sentence should be:
John (is one of those) bank presidents who are always worrying (about) making loans to young married couples (with no) established credit.