Delay of the move-in date already before the start of construction

It's a lazy excuse. Corona is not unexpected, another lockdown is not to be expected. Disrupted supply chains have been completely normal in construction for at least 1 year. Since you don't have a building permit yet, I would be surprised by a fixed scheduling.

I would do the following now:
1. Call and ask how much it concerns
2. Make it clear in a friendly but firm manner that it is not your problem if the general contractor overcommitted three months ago with his orders and that you do not like the way the announcement was made.
3. If it’s only about 1-2 months, don’t take it too seriously. But take the new date bindingly as a contractual addition, if necessary with an adjusted (=increased) contractual penalty.

I feel the same way. I would also be surprised if the construction project is already delayed today. At least it cannot be because of our construction project.

Your suggestion for the procedure matches my intention. However, I am still not sure whether it is legally wise to just accept this letter as is or whether I should rather inform them in writing that I do not simply agree to it. In principle, the letter does not change anything about my contract and the agreed date, does it? After the move-in date has passed, the general contractor would be directly in default anyway (contract according to the Building Code) and would need to prove that he is not responsible for this. This letter should not change anything about that either. If it does, then any reaction from me now needs to be carefully considered.

In principle, the general contractor "deletes" the move-in date from the contract with the letter and postpones it indefinitely. So it could be three months or years. Legally, this should not be so easily possible. I would almost go so far as to say that this letter has no legal significance for the contract.

As you say, you can talk about everything and one or two months is not bad. I also don’t see it so strictly. But I find such a letter without comment difficult. After two months of delay, I would then enter the provision of interest payments on the outstanding installments.

I wanted to take out a builder’s legal protection insurance anyway—I’ll do that tomorrow :D

That is definitely too late, at least 6 months before the contract is signed. Maybe the guy will simply just leave if he can postpone without getting into trouble. That is why I would have a professional letter drawn up.

That is definitely too late, at least 6 months before signing the contract.

Maybe the guy dares to just cancel if he can postpone without getting into trouble. That’s why I would have a professional letter drafted.

It is not too late. With our insurance possible without waiting period, at the latest before the start of construction. The contract is available.

It also seems to me like "I'll send the letter out and see who complains, then we will sue them." A professional letter would then be from a lawyer, maybe with some luck that would already be covered by legal insurance.

I wanted to take out a builder's legal protection insurance anyway - I'll do that tomorrow :D

Have you already looked into a builder's legal protection insurance in more detail? We originally wanted to do that too, but then decided against it because, despite what we considered a very high premium, almost every case we could think of was excluded.

Before reacting in any way, I would consult a lawyer for advice to avoid making any formal-legal mistakes. The specialist lawyer for construction law can tell you exactly how to respond, what to write and what not to write. However, I would keep the lawyer in the background. Such advice doesn’t cost a fortune, but may save you from pitfalls or even damage.

Of course, I don’t know your contract. Therefore, this is all speculation, my first thought was that the [GU] wants to get out of the contract and gives you a reason to end the collaboration, so that he is not the one doing it. But as I said, just my thought.

What kind of insurance is that supposed to be?