Warranty period and Covid-19? Extension of the period?

I would have had it done by a professional!

I am not sure whether the notification by email is sufficient or whether it must be registered in writing by documented letter.

Why should that not be enough? An answer has obviously been given, so it is clearly recognizable that the message has been received.

If there is already contact with the developer anyway, I would simply get a written confirmation from him (letter with signature) stating that the defect was reported before the end of the warranty period and that the repair will also take place within the warranty scope after day xyz (insert the end of the warranty period here). This way you play it safe, even if the warranty for the reported defect should be suspended (I'm not an expert, just a guess).

The statements that it is not possible because of Corona – well. Just ask if at his subcontractors even one person is working on the construction sites at the same time. I think they are trying to stall you here. Such a meeting is clearly to be classified as work-related and not as a private Corona party...

This is now, of course, a legal issue. But property. the limitation period is suspended if negotiations between both parties lead to a defect. This could be argued in your case – you say a defect is present. The contractor initially says, I don't believe that and I need to look at the problem with my specialists first.
But I am not a lawyer...

I also think that should be enough for a suspension of the limitation period (§ 203 Building Code). The Federal Court of Justice (BGH) says the following about this:
"The creditor only needs to make it clear that he is asserting a claim and what he bases it on in order to assume negotiations. Once this happens, any exchange of opinions about the damage or another fact that may be the subject of claims is sufficient, unless every claim is immediately and clearly rejected." BGH IX ZR 233/17

If you have documented the responses of the construction company in writing (email, letters, etc.) and not everything was done by phone, that should be enough for a suspension. If the construction company eventually resists, you may have to act quickly: "If negotiations are ongoing between the debtor and the creditor about the claim or the circumstances justifying the claim, the limitation period is suspended until one party refuses to continue the negotiations. The limitation period only begins at the earliest three months after the suspension ends." (§ 203 Building Code)