Big update:
I got a rather lengthy mail from the debt collecting agency. It boils down to this:
1) Even though there's no signed contract, and the contract they sent me stated that ,if I don't sign it within two weeks, the contract will be automatically invalidated, the contract is still valid.
2) Because the contract is valid, I owe them a month's worth of rent for the apartment I never rented. They view my deposit for the apartment I did rent as partial payment for that. My deposit was €820,-. The imagine there being a rent of €897,- for the apartment I didn't rent, ergo, I owe them €77,- for that and my deposit is voided. Keep in mind that the contract specified a rent of €857,- (so 40 less) and that bollocks price was exactly why I didn't sign the contract in the first place.
3) I didn't pay my last month's rent for my old apartment. (I did; he has the receipts for this). I owe them €820 for that.
4) They sent me an overview for the usage of water, gas and electricity. According to them, I owe them €407,- for that. I checked the meter readings they sent and compared it to the pictures I took of the meter readings. I'm not sure what they did, but it's absolutely false.
5) In total, they demand €1,229.67. If I don't pay, they'll sue me. They'll then also sue me for all the damages done by someone else to an apartment I didn't rent. They're also suing my mum.
All in all, I found this rather funny. Hence, I replied by only sending them this picture of me:
Edited 7/23/2020 10:58:40