Hello,
I am not allowed to provide legal advice here, as this is exclusively the domain of consulting professions. Therefore, please understand my answer as a "personal opinion."
Is the general contractor allowed to stop construction without notice?
Yes, you have breached the contract unilaterally by not transferring the due payment for services rendered.
especially if I use the documented defects of the Verband Privater Bauherren as a reason for withholding payment and have offers/cost estimates for the renovation.
Offers for renovation do not interest anyone at first, because your contractual partner has the right to remedy defects. Only when a reasonable deadline passes without result may you commission a third party!
Unfortunately, you have an unfortunate combination of circumstances: on the one hand, a lawyer you engaged and a Verband Privater Bauherren advisor, and on the other hand, a general contractor who presumably made mistakes. In such situations, it is not uncommon that the problem is recognized but the willingness to develop solutions is neither visible nor present; all the more so if the owed payment for services is withheld. It is necessary for the parties to sit down at a table and talk... talk factually... talk problem-oriented... without mutual accusations; the goal is not to search for problems but to solve them.
From my point of view, your Verband Privater Bauherren advisor bears a special responsibility; I hope he can hold back? In my opinion, he _must_ act as a mediator, not construct threats with a wagging finger. He should take the pressure out of the situation, find a common _language base_ with your general contractor, and equally, you need to hold back. Because –
a) missing functional potential equalization/foundation earth electrode
is bad, no question, but not unsolvable
Everything else reads more like a slogan, in the typical vocabulary – if you have not formulated it yourself – of an association expert. Not exactly motivating for discussions, but by no means unsolvable.
Conclusion: the better educated, professionally and life-experienced the Verband Privater Bauherren advisor is – the higher the chance to complete the contract as owed and not have to settle opposing positions in lengthy court proceedings. Keep in mind – in house construction, parties always meet before the regional court… that takes time… costs a lot of money/nerves/time and usually ends in a settlement.
Best regards from the Rhineland