Calculate Expression does not work when the third value is missing

Could you set these sum properties to default to 0 if no invoices of that category are found? Then the calculation should always succeed I think.

Where can I set this up? Is that on the Metadatacard configuration or by the Calculation (PropertyCalculator)

Either might work, at least if these objects are always created in the client and not via the API since metadata card configurations are only applied when the metadata card is displayed to the user. You could test SetValue in MDCC first since it's probably easier to set up.

The problem is that the calculation does not allow any value, the values are extracted from the linked calculations and then added together for each category and a new construction project I cannot create, because these are linked to a database.

You'll need to find some way to make sure there's a value for all the properties that Property Calculator uses in the operation. One option would be to use three helper properties that you can set yourself. I.e. copy the calculated values to helper properties and if the value is empty, set it as 0. Then use these helper properties in your Property Calculator sum operation.