Hi Fezder, On a more serious note, let me see if I understand your predicament. You have used some terms that seem to have lost something in translation. So I will try to describe it in my own words:
You rent an apartment/dwelling and are a little behind on your rent. 400 euro in the US would be about 1 month's rent, maybe two. You got a letter from a debt collection company that you had never heard of. If that is correct, the good part is that you hadn't heard of the company before, so you probably don't have terrible credit. The bad is that the landlord turned you over to an expert to collect the rent. That has already cost the landloard 30 to 50% of the amount due. He will not be happy with you regardless of the outcome.
In the US, there are strict laws on what a debt collection agency can do, BUT you need to assert your rights before many of those laws have teeth. For example, the agency may contact your family, friends, employer and so forth, unless you send it a demand letter (usually, but not necessarily by an attorney) telling the debt collector not do make any further contacts. Once that letter is received by the debt collector, further contacts of that nature can expose the collector and landlord to serious liability.
Real Estate law in the US is very complex and varies significantly by state. I suspect such laws in Finland are equally complex because of the state's interest in protecting both investors' interests and renters from being thrown out into the cold to become burdens on the state. The simple advice is to see an attorney. But that can be expensive. I suspect there are government agencies that can advise you, but not represent you. I suggest seeing such an agency. If you were in the US, I would suggest getting that cease and desist letter sent tomorrow, if not sooner. There are horror stories of innocent people getting caught in the credit reporting mess that comes back to haunt them many years later. One important thing to be able to show in US courts is that you have made and are making a sincere effort to pay what you owe.
BTW, sending such letters just before a holiday is not accidental. It is a calculated, psychological tactic to make you feel off balance.
Good luck.
John