Nextcloud install in a non-webroot directory which is actually subdomain root

Hello, I’m trying to set-up Nextcloud on a shared hosting instance.
The webhost doesn’t “natively” support multiple domains, so I have a .htaccess file in my root that rewrites the URLs to subfolders in the web root based on the Host header, as follows:

RewriteCond %{HTTP_HOST} ^$ [NC]
RewriteCond %{REQUEST_URI} !^/*$
RewriteRule ^(.*)$  /$1 [L]

I downloaded and ran the setup-nextcloud.php script, and it installed just fine, except that when I try to access, I get an internal server error, and this in the logs:

Exception: The requested uri(/) cannot be processed by the script '/')

I have to go to for it to work, then everything works fine, URLs are generated fine, etc. I’d however prefer if the Nextcloud instance looked as if it truly were in the root.

I’m not sure on which side of the config the error lies (the .htaccess or the config.php), but fiddling with neither acheived any results. Furthermore, given that I’m on shared hosting which does not offer SSH access, I can’t run occ.

Any ideas? Thank you!