When retrieving the "Steuerschlüssel" with DATEVconnect via the resource accounting-transaction-keys, the tax rates (tax_rate) valid since 01.07.2020 are not delivered. Instead, the old tax rates are displayed. The problem will be solved with the next service release (13.1) on 13.07.2020. Please check if you are using this resource and if necessary store the correct tax rates directly in your software.
You can find the current tax key including tax rate at https://www.datev.de/info-db/1018040.