Nextcloud, increase inotify limit.

Add

fs.inotify.max_user_watches = 100000
….to
/etc/sysctl.conf

…or rather create a new file called something like
/etc/sysctl.d/somethinglike.conf

…and in that file put the
fs.inotify.max_user_watches = 100000

Your nextcloud client should then stop complaining.