[first post after lurking for a long time]

I rented a car from Sixt in Slovenia (Sunday June 16, from Ljubljana train station to Ljubljana airport).
I only had the car for a few hours and had no accident whatsoever.

Upon pickup, I signed that everything is fine. Upon return, I gave the keys to an attendant, who then told me that I can go.
Even though I returned the car during business hours, they did not do the return inspection and paperwork right away.

Now 2 days later, they sent me the return paperwork by email including a damage assessment claiming I caused a scratch on the car ("5 cm scratch down to primer").

They demand I fill out a damage report now, where I have to describe how the damage occurred, even though I did not cause it.
They also threaten if I don't fill out the damage report, I will be liable for even higher repair costs.

Any experience in handling this?

I've never had a problem with other rental car companies. Is this a general problem with Sixt?