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You have an AAAA DNS record set for your domain, pointing to a non-functional IPv6 address. Let's Encryp prefers IPv6 over IPv4, so a non-functional IPv6 address will cause issues.
Not directly, no. However, with some basic debugging, it can be found.
First, I ran curl -LIv http://www.byte-store.de/.well-known/acme-challenge/VZUnkNmu-t2uWP1QXN68rvzFIa9CfCxsKUX84Th77mg, which gave me a successful result actually. However, I noticed curl outputting:
cURL will actually succeed, as it'll fall back to IPv4 by default (unless you force IPv6 with the -6 option), but Let's Encrypt doesn't for these kind of problems (timeout). It will reconnect with IPv4 when IPv6 gives a "connection refused" I believe.