Alright, go back to 192.168.1.21 and start up Apache again.
In short, your problem is that Apache configured to only serve Nextcloud to people requesting 192.168.1.21:
When Caddy is proxying from ncloud.skywave.com with the transparent preset enabled, though, it’s requesting ncloud.skywave.com from Apache, not 192.168.1.21.
You can either:
1. Tell Apache to listen for ncloud.skywave.com
Edit /etc/apache2/sites-available/nextcloud.conf, add a ServerAlias:
ServerName 192.168.1.21
ServerAlias ncloud.skywave.com
2. Tell Caddy to request 192.168.1.21
Either add header_upstream Host 192.168.1.21 to your Proxy directive, OR remove transparent. This is a bit of a hack and comes with some downsides.