Solved by restarting my FritzBox. something with the DNS must have been wrong…
Same problem here: App Site not available
Edit:
Problem came back again. This time a restart of the FritzBox did not help…
I changed the DNS Server in the FritzBox to
IPv4: 8.8.8.8 and 8.8.4.4
IPv6: 2001:4860:4860::8888 and 2001:4860:4860::8844
Now I can reach out to apps.nextcloud.com again. Definitely a FritzBox problem…