Hello,
I running for some month nextcloud on my server but suddenly the AppStore ist mit visible. I setup an test instance and it works at the same server. It begins with an warning that the internet connection is not available.
Is it possible that some settings are saved at the database? Curl -i test works, no issue here. Has anyone an idea?
Check your logs
Error log say (not a directory) .
2017/01/18 21:11:44 [error] 22133#0: *5961 “/var/www/vhosts/larsmueller.net/nextcloud/index.php/apps/activity/index.html” is not found (20: Not a directory), client: 5.10.188.170, server: cloud.larsmueller.net, request: “GET /index.php/apps/activity/ HTTP/2.0”, host: “cloud.larsmueller.net”
Probably unrelated, anything else?
i found my issue 
i moved the cloud to an other subdomain and i forgot to clean the cache tables at the database.
After that i works again.