If you have discovered an error in the utility bill, you should not get upset right away. First ask the landlord, you also have the right to see receipts. You should inform the landlord in writing of any specific objections and ask for a statement.
Tenants also have one year to raise objections after the ancillary cost statement has been delivered. If no agreement can be reached in a direct exchange, you should consult a lawyer. Since a legal dispute can drag on for a long time, you are on the safe side if you make the required additional payments with reservations. (with material from dpa)