wpml-string does not take translation

Hi there,
some months ago I have translated a sentence of the invoicing plugin with the wpml string translation into German (see attachment). For some weeks now I have been trying to change the German wording but the system simply does not take any changes. On Checkout-pages and invoice emails it is still the old Germany wording displayed. Thisis strange, after all I myself once have translated this string and now I cannot get hold of it anymore.

My questions:
How can I change the German wording now?
I cannot find this string in the Invoicing po-file on the server.
Would it make sense to download the po from wpml and somehow add it to the po-file on the server? What would I have to do then?

Many thanks in advance
Birgit