Vaillant Service Caused Flue Leak After Maintenance on My Eco Tec Pro Boiler

I paid ₺4715 for the routine maintenance of my Vaillant Eco Tec Pro boiler. After the maintenance, I started experiencing issues with water leaking from the flue. I contacted the service again to report the problem, but not only did they fail to fix the issue they caused, they also demanded an additional service fee. If the service cannot resolve a problem caused by their own mistake, what is the point of their job? I absolutely cannot recommend this brand to anyone. We have been left in a difficult situation. My boiler was working perfectly and had no issues before the maintenance. If this problem leads to any malfunction, fire, or explosion, I will take legal action to defend my rights. I expect this issue to be resolved as soon as possible.