sixt fraudulent damage claims , Internet

I rented a car with Sixt through the Hotwire website while i was in Vienna. Upon returning the car and checking out at the front desk, the agency informed us of "scratches under the bumper," which obviously were not visible. But since we were covered by the Hotwire insurance, the agent told us it was fine and sent us off after acquiring my signature on an electronic platform. Months later, after returning back to the states, I got the notice in the mail to fill out an accident report. Since I had been away, it took time before I was able to return the accident report. After which, a bill requesting to pay for the damages of EUR 622 came, which is now escalated to EUR 1,010, and threatened to take further by legal actions. We never saw the alleged damage. If I have not seen picture or any evidence that the car was damaged right after the return, then how could it be possible that someone else did not inflict the alleged damages? 

I have had numerous contacts with agents from sixt, but they were not able to articulate or explain why it is the insurance we purchased through Hotwire was not taking care of the alleged damage. Nor proof that i had inflicted the damages. They have one document in german with my signature, which was lifted from the electronic version. i have not signed anything that i did not understand, particularly in german. i have contacted Hotwire about this issue, but due to the fact it was booked outside of the states, they were not able to help me other than rerouting me to their contractor, car crawler in europe, which said that they were not set up to follow up and dispute claims.

I've found numerous accounts of customers with similar experiences of being charged fraudulent damages from Sixt: http://www.complaintsboard.com/?search sixt