@foomep: do you also have push enabled in the iOS settings?
I think you need z-push for push to work with Nextcloud. Do you have z-push installed on your server?
I don’t use z-push myself, so I disabled the push setting in iOS and let the accounts refresh every 30 minutes (which is often enough for my use case). This works for me reliably.
Can you try disabling the push setting, set it to a time interval instead and see if this solves your problem?