Short update:
The lawyer says the same as I and some others here. He no longer sees force majeure in Corona, we should clearly object in writing and not accept the delay, while also pointing out the contractual penalty. In addition, we should state what burdens us due to the delay, e.g. provision interest, etc.
The phone call with the managing director was very pleasant, I can understand the situation regarding the steel shortage, etc., it may also be that our construction project will not be affected by delays. The letter was apparently a reaction to supposedly "legally" protect themselves. Another builder apparently caused quite a bit of stress because their construction site was not finished. The price remains of course, and I was assured that they would do everything possible.
Now it’s a matter of waiting to see when it actually starts. It is important that we have clearly and in writing expressed our position. According to the lawyer, the contractor’s letter does not easily override the contractually agreed date.