Continuing NS8 Nextcloud + other problems

I’m sorry to read about your bad migration experience. I think it’s possible to fix it.

Did you set up the notification settings in the NS8 settings? See also Email notifications — NS8 documentation

It seems that the app “antivirus for files” is enabled, please disable it and test again. I could reproduce the missing preview by enabling the app but Collabora still worked here.

If the preview still doesn’t work you could try other scale or quality settings, see also Previews configuration — Nextcloud latest Administration Manual latest documentation

What websites is it about?
Which third party app? lamp, wordpress or the webserver app.
What did you already try?