I'm in a similar situation, but in my case it's a gas line. So I looked into the subject a bit... to put it briefly: You do not have to pay for the relocation.
Reasoning:
1. Basically, you can rely on the correctness of the land register. If no encumbrances are recorded at the time of purchase and it cannot be proven that you knew about the line before the purchase, then it must be assumed that what is stated in the land register is correct.
2. If the line was laid before the reunification, energy suppliers could simply register the line rights in the land register until, I think it was 2010. I guess TEN missed this deadline. As long as the property has/had the same owner, the right of way for the line exists even without entry in the land register. If the property is sold and the right of way is not officially secured... then the energy supplier is out of luck and must relocate the line.
3. It's also worth taking a look at the Low Voltage Ordinance. More precisely, §12 is worth checking out. It actually pretty clearly regulates what you as the connection user must tolerate and what not, or rather who has to pay for a relocation. In short: if it’s not your own connection, then no.
I do find it remarkable that the TEN initially want to charge you for it. With my network operator, there was no hesitation. They are now planning a new route with a detour of several hundred meters just because of me...