Your accountant showing up again.
I don't know anything about the Croatian legal system, I know a lot about how the US legal system deals with debts and torts.
In the US it is common practice (frequently involving smaller amounts of money) to send a "demand letter' outlining the nature of the problem and the proposed cure. Example "Your kid threw a rock through my window and did five hundred dollars damage, invoice for the repair included. Please pay this amount within thirty days to avoid further action." This puts the defendant on notice, if the case goes to court the plaintiff can point out they made a reasonable effort to settle the problem outside the courts.
I think this letter should be read as a demand letter not a threat. It is poorly worded (a good lawyer would try to sound friendlier) but otherwise not that far off the mark. One telling factor is the low ball statement of damages. In the US $3K (or whatever it was) is chump change, a matter for small claims court. No lawyer with brains is going to give up the big money coming out of the starting gate.