Images broken or giving 403 error on multisite subdomain

So I’ve searched all over for solutions to this, and have tried recommendations I’ve seen to others with similar problems, but am still at a loss.

Images on a multisite subdomain are not showing up. More specifically, they do not show up on pages/posts or in the media library. If I go all the way in to “Edit Image” in the library, I can see it. If I pull up the image file itself in the browser, it works (sometimes) or brings up a 403 error.

Here is my .htaccess file – I believe it should be correct:

# BEGIN WordPress
<IfModule mod_rewrite.c>

RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]

# uploaded files
RewriteRule ^files/(.+) wp-includes/ms-files.php?file=$1 [L]

# add a trailing slash to /wp-admin
RewriteRule ^([_0-9a-zA-Z-]+/)?wp-admin$ $1wp-admin/ [R=301,L]

RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ - [L]
RewriteRule  ^([_0-9a-zA-Z-]+/)?(wp-(content|admin|includes).*) $2 [L]
RewriteRule  ^([_0-9a-zA-Z-]+/)?(.*\.php)$ $2 [L]
RewriteRule . index.php [L]

</IfModule>
# END WordPress

I double-checked permissions – all folders/directories are 755, and the image files themselves are 644.

I also contacted my host to check about having AllowOverride All in my httpd.conf file – however, I was told I could do that myself by adding the following into my .htaccess file:

<Directory "Account path">
AllowOverride All
</Directory>

I did that (with the account path info where indicated), but that just broke my site and brought up a 500 error when trying to get to any pages on it.

There’s been some strange behavior – like images showing up sometimes in the post editor, but being broken most of the time, and images on particular posts showing up for a little while, but then going back to broken.

My host’s most recent suggestion is that the WordPress upload path might be incorrect…

Any help would be greatly appreciated!