I wonder if the blanket rewrite to /SOgo{uri} is doubling up when the upstream app returns a link to /SOgo{uri} (and Caddy then rewrites to /SOgo/SOgo{uri} etc). That would explain the index HTML working but assets being broken.
As a bit of a guessed shot, you could try limiting the rewrite to prevent it from doubling up. Something like:
@not_SOgo not path /SOgo*
rewrite @not_SOgo /SOgo{uri}
Or maybe it even makes more sense to redirect, honestly? Kinda depends.
Other than that, yeah, you’re gonna need to fill out the full template for us to give you better help.