I recently stumbled across the sentence below, but I'm not sure which version is correct (or more specifically, I personally am pretty sure it's the second one, but other people disagreed):
The problem is the rules.
or
The problem are the rules.
I would argue that the second version is correct, as I think "the rules" are what dictate the conjugation here. If the sentence were flipped, "The rules is the problem" is very obviously incorrect (as far as I can tell).
Curiously, when using DeepL to translate this sentence from my mother tongue (German) to English, I'm given the option between is/are, but "is" is the default suggestion. However, when translating from EN to DE, I don't get an option to pick the singular "ist", which makes sense, as that would most certainly be incorrect (but English grammar might be different, hence the question).
Even more interestingly, when translating the flipped sentence to EN, I also get both options, but the default is "are".
What is, therefore, the correct conjugation for the sentence above?