If that’s your entire Caddyfile, then:
The 404s are generated because Caddy has no site definition for
levelthreemage.de - it only has a definition for
I’d wager that the 500s are linked to the fact that you’re putting Nextcloud in a subfolder. I believe you can configure an overwrite of the web root, but you’d need to look at the Nextcloud documentation. You might not need to fiddle with the rewrites, but I’m not 100% sure.
My strong suggestion is to drop the path from the site label (i.e.
levelthreemage.de) and see if that solves the 500 series errors (I fully expect it to solve the 404s). If it does, you’ll know that the obstacle is figuring out how to get Nextcloud to play nicely under the subfolder.