I have received an invoice from Epass24, even though I had a valid toll tag.

Please get in contact with the supplier with which you have signed agreement/tag with in order to check why the tag was not valid at time of your passages. If the tag where active or connected to your vehicle licenseplatenumber at time of passages, it would have been charged directly via the tag/agreement and not been invoiced via Epass24. Therefore this has to be investigated and resolved via your toll tag provider.