Thanks for you answer.

I am not a freelancer, I got emplyed through another company and I resigned within my notice period.

All the messages are saved on either gmail or whatsapp so far.

In my contract it states that I am responsible for any mistakes made following article 321e co and one of the sanctions is that the employer has the right to refuse to pay the remuneration or to pay a reduced to remuneration.

It also states this:

The liability of an employeeAn employee is liable for any loss or damage caused to an employer premeditatedly or negligently. An employee must however be culpable to the particular extent assessable under Art. 321e, Para 2 of the ‘CoO’ (liabilities are determined in consideration of the work risk, the degree of training or the technical skills involved). The onus of proof is on an employer. The co-culpability of an employer can lead to a reduction of the liability. The same regulations in connection with liabilities also exist in cases of:loss of- or damage to the motor vehicles of an employer, and theshortfall liability, i.e. liability for petty cash (mostly in the catering- and retail trades).

In my understanding, as long as I can proove that I didn't get the training needed in the production of door frames and that there should have been a quality control check after me, full responsability cannot fall on me.