Learn German: Es wurde heute verhandelt, wobei die Nebenkosten ein Problem waren. - There was negotiating today, whereby the utility costs were a problem.

The word wobei translates to 'whereby', 'in which case', or 'during which'. It connects the two clauses, but instead of pointing back to a single noun, it refers to the entire action of the first clause (the negotiating). Since it introduces a dependent clause, it forces the conjugated verb (waren) to the very end.

Why is the first clause written as 'Es wurde heute verhandelt' without a real person as the subject?

This is an impersonal passive construction. By combining the dummy subject es with the passive voice (wurde ... verhandelt), the sentence puts all the focus on the action itself rather than who was doing it. It translates smoothly to 'Negotiations took place today' or 'There was negotiating today.'

Could I just use 'aber' (but) instead of 'wobei' here?

You could use aber, but wobei adds a slightly different nuance. While aber simply contrasts two independent ideas, wobei frames the utility costs as a restricting detail or condition that specifically arose during the negotiations. Also note the grammar difference: aber keeps normal word order, whereas wobei pushes the verb to the end.

Why is the verb 'waren' plural if the clause ends with a singular noun ('ein Problem')?