Seit kurzem steht in der selbstgehosteten Version (4.5.9) statt Kundennummer dort Steuernummer.
Recently, the self-hosted version (4.5.9) has been replaced by a tax number instead of a customer number.

Our translations are managed here: https://www.transifex.com/invoice-ninja
Thanks.
This "bug" is still present in the 4.5.14