Quality problem Hansgrohe faucet, warranty claim

Our Hansgrohe faucet on the washbasin in our new apartment became stiff within 1 year, which is actually covered by the warranty. A technician from Hansgrohe's customer service company came for the repair and found that the problem was caused by the wrong cleaner and installed a new cartridge because on the day of the repair I had used the vinegar cleaner from [DM-Markt] and he said that only citric acid-based cleaner should be used for the faucet and the care instructions are stated in the supplied user manual. But I received neither the user manual nor any note from the builder, which the builder also admitted. Afterwards, I received an invoice for 300 EUR from the builder because the misuse was my fault. However, the problem reoccurred within two months, although after the first repair I only used citric acid-based cleaner. This time I contacted Hansgrohe customer service, but the company said that I should send the defective cartridge to them and have it checked there to see if any cleaning agent or whatever had gotten into the cartridge. If yes, then it is again not covered by the warranty. This means that I am definitely at fault again and have to pay for the second repair myself. But why did the water with cleaning agent only get into the cartridge of this faucet? How can this happen when I have maintained all faucets the same way? What is the use of further repairs if the faucet will become stiff again after some time and Hansgrohe will refuse the warranty again? With so much money I can already buy a new faucet from another manufacturer.
Does anyone have similar experience like mine? What could be the cause for the stiff faucet besides cleaning? I really do not know what to do now because Hansgrohe apparently can just find any reason to deny the warranty. I can no longer trust this company, which in my case takes no responsibility for its own product.
I ask here for your help and advice. Do I have to pay the first invoice even though the first repair essentially failed? What should I do now to make it right? Or is simply no further repair and staying away from Hansgrohe the best solution?
I hope I have clearly written my problem as a foreigner. Anyway, thank you very much for your effort in reading my long text. I thank you in advance for all tips and suggestions!!!!

Quote from another Hansgrohe customer: [HR][/HR] Registered since 05.10.2012 Posts 2
This is how Hansgrohe deals with quality defects and warranty claims During a bathroom renovation in May 2011, a Hansgrohe shower valve (Mediano shower thermostat battery 142261) was supplied and installed by a professional company. Now, in September 2012, we noticed the chrome layer peeling off the right temperature control knob. We noticed this because my wife, while showering and adjusting the temperature, cut her finger deeply on the razor-sharp edge of the peeling. We reported the damage to the professional company. They forwarded the damage to Hansgrohe. According to the professional company, Hansgrohe rejected the warranty claim, claiming: we treated the valve with unauthorized cleaning agents. An outrageous impudence. This statement was made without speaking to us or inspecting the valve. This valve was never treated with any cleaning agents, only wiped with a soft cloth after showering. With these nasty tricks, Hansgrohe tries to dismiss a warranty claim arising from a quality defect.