[Muromec]

>If you own a copy signed by you, and a copy signed by another person, as we have the proof of the content of each one, you know that it is the same contract that was agreed...

Then you need to show a third party the contract signed by both sides. On paper or sent by fax. When you say digital document trust services eIDAS, the party replied that it's not a participant in the trust architecture (as defined by eIDAS-implementing law) and didn't buy the license for software needed to verify it. The party is a judge, so they are right and you are wrong. True story that did actually happen and nobody was even disputing the fact of document being signed.