the nextcloud server is not available to me in view of server. other target hosts/websites work without any troubles. only nextcloud servers got the problem.
a ping to apps.nextcloud.com works. IPv4 and IPv6!
a simple php curl fetch shows this:
* Trying 176.9.217.53:443...
* TCP_NODELAY set
* Trying 2a01:4f8:130:32f1::53:443...
* TCP_NODELAY set
* After 4999ms connect time, move on!
* connect to 176.9.217.53 port 443 failed: Connection timed out
* After 2399ms connect time, move on!
* connect to 2a01:4f8:130:32f1::53 port 443 failed: Connection timed out
* Failed to connect to apps.nextcloud.com port 443: Connection timed out
* Closing connection 0
Nextcloud Log itself:
{"reqId":"VYHy88UhvQtmVhuohGX9","level":2,"time":"2020-10-10T14:58:30+00:00","remoteAddr":"91.0.xxx.xxx","user":"Admin","app":"appstoreFetcher","method":"GET","url":"/settings/apps/list","message":"Could not connect to appstore: cURL error 28: Failed to connect to apps.nextcloud.com port 443: Connection timed out (see https://curl.haxx.se/libcurl/c/libcurl-errors.html)","userAgent":"Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:81.0) Gecko/20100101 Firefox/81.0","version":"20.0.0.9"}