Apache 2 .htaccess matching all sub directories of current directory

I've seen this when serving files from SMB- or CIFS-mounted shares. Try turning off sendfile support using:

EnableSendfile Off

You can do this in the main server configuration, .htacces file or in a vhost/directory block.

If you are willing to use a directory prefix instead of "/", you can use something like this:

# Let's setup WebDAV first
<Directory /var/lib/storage>
        Dav On
        Options Indexes
        AllowOverride None
</Directory>
# Now we'll set up the user area mapping
RewriteCond %{REQUEST_URI} ^/storage/
RewriteRule ^/storage/(.*?)$ /var/lib/storage/user/%{LA-U:REMOTE_USER}/$1 [L]

Moreover, the same user directories can be accessed read-only using the /~user/ syntax

# Public area can be accessed as https://server/~user/
RewriteCond %{REQUEST_URI} ^/~
RewriteCond %{REQUEST_METHOD} ^(GET|POST)$
RewriteRule ^/~([^/]+)/?(.*)    /var/lib/storage/user/$1/$2 [L]

YMMV