Hi all,
this is just meant as a tip in case you originally followed the ownCloud documentation <= 7.0 and upgraded and migrated to Nextcloud later.
Until then ownCloud recommended within their documentation some “performance enhancements” set via sysctl. Among them was also net.ipv4.tcp_tw_recycle=1.
TL;DR: do not enable net.ipv4.tcp_tw_recycle.
If you want to know more about and why, see here: Coping with the TCP TIME-WAIT state on busy Linux servers
man tcp(7) explains why it is a bad idea:
Enabling this option is not recommended since this causes problems when working with NAT (Network Address Translation).
As a consequence, if you have one client behind a NAT that connects to Nextcloud, no other client behind that same NAT can connect to the Nextcloud server anymore. 
It took us some time to find the culprit. ownCloud removed the recommendation some time ago: remove linux tuning tips · owncloud-archive/documentation@5ba5a6d · GitHub but we didn’t follow this change in the documentation…