Cause
The codes for handling VAT according to the Flat tax rate method are misssing from the VAT codes table.
Solution
Check the code setting in the VAT codes table. At least one code containing one of the following numbers is required: 320,321,330,331 (the figures are specified in the GR1 column). The amount type must be 0=VAT included. Refer to Swiss VAT tax report - Flat tax rate method for further information.