403 errors for images only on newly added ServerAlias - apache

Question

I have had two webservers running on the same machine with addresses like the following:
internal.myservername.com
and
test.myservername.com
The test site is (on some pages) pulling images from the internal site, and works flawlessly. Now that we are out of testing, I wanted to add/change the test domain to be the main site, so I added the apache server alias of myservername.com to the test site, but although all pages work, any images from internal will not show up on that address, resulting in a 403 error:
[access_compat:error] AH01797: client denied by server configuration
I have also tried setting up myservername.com as its own domain, and also tried making it the primary name on the test site (with test being an alias). In all cases, test.myservername.com continues to work fine and display images correctly, whereas myservername.com will not (resulting in all the 403 errors for these embedded images).
I have checked all configs and find nothing amiss. And there is nothing in my .htaccess files or conf files that is specific to "test". I am stumped.
I should probably also mention that I am running on a Mac OS Sierra server. (Server version: Apache/2.4.23 (Unix))
UPDATE WITH REQUESTED DETAIL:
img tags: there is nothing special in the image tags, they are called using the full url, ie
<img src="http://internal.myservername.com/images/imagename.jpg" />
I should also add that if I take the exact src url and paste it alone into a browser, the image WILL load. It is only embedded in the page that it will refuse to load. (and only on the main domain, test works fine embedded in img tag)
And the setup of test/main conf is:
<VirtualHost 127.0.0.1:34580>
ServerName http://test.myservername.com:80
ServerAdmin admin#example.com
DocumentRoot "/Library/Server/Web/Data/Sites/myservername.com/plugins/mywebsite"
DirectoryIndex index.php home.php index.html
CustomLog /var/log/apache2/access_log combinedvhost
ErrorLog /var/log/apache2/error_log
<IfModule mod_ssl.c>
SSLEngine Off
SSLCipherSuite "HIGH:MEDIUM:!MD5:!RC4:!3DES"
SSLProtocol -all +TLSv1 +TLSv1.1 +TLSv1.2
SSLProxyEngine Off
SSLProxyProtocol -all +TLSv1 +TLSv1.1 +TLSv1.2
</IfModule>
<IfModule mod_secure_transport.c>
MSTEngine Off
MSTCipherSuite HIGH, MEDIUM
MSTProtocolRange TLSv1.2 TLSv1.2
MSTProxyEngine On
MSTProxyProtocolRange TLSv1.2 TLSv1.2
</IfModule>
<Directory "/Library/Server/Web/Data/Sites/myservername.com/plugins/mywebsite">
Options All -Indexes +ExecCGI +Includes +MultiViews
AllowOverride All
<IfModule mod_dav.c>
DAV Off
</IfModule>
<IfDefine !WEBSERVICE_ON>
Require all denied
ErrorDocument 403 /customerror/websitesoff403.html
</IfDefine>
</Directory>
ServerAlias myservername.com
And the conf for internal:
<VirtualHost 127.0.0.1:34580>
ServerName http://internal.myservername.com:80
ServerAdmin admin#example.com
DocumentRoot "/Library/Server/Web/Data/Sites/myservername.com"
DirectoryIndex index.php
CustomLog /var/log/apache2/access_log combinedvhost
ErrorLog /var/log/apache2/error_log
<IfModule mod_ssl.c>
SSLEngine Off
SSLCipherSuite "HIGH:MEDIUM:!MD5:!RC4:!3DES"
SSLProtocol -all +TLSv1 +TLSv1.1 +TLSv1.2
SSLProxyEngine Off
SSLProxyProtocol -all +TLSv1 +TLSv1.1 +TLSv1.2
</IfModule>
<IfModule mod_secure_transport.c>
MSTEngine Off
MSTCipherSuite HIGH, MEDIUM
MSTProtocolRange TLSv1.2 TLSv1.2
MSTProxyEngine On
MSTProxyProtocolRange TLSv1.2 TLSv1.2
</IfModule>
<IfModule mod_headers.c>
<filesmatch "^.*www.*\.jpg$">
Header set Cache-Control "max-age=2678400, public"
</filesmatch>
</IfModule>
<Directory "/Library/Server/Web/Data/Sites/myservername.com">
Options All -Indexes +ExecCGI +Includes +MultiViews
AllowOverride All
Require all granted
<IfModule mod_dav.c>
DAV Off
</IfModule>
<IfDefine !WEBSERVICE_ON>
Require all denied
ErrorDocument 403 /customerror/websitesoff403.html
</IfDefine>
</Directory>
</VirtualHost>

Answer 1

UGH, I just figured out what was causing this, and here are the steps I took to figure it out:
Because my error was happening in mod_access_compat, I turned off that module to see if anything changed
This resulted in a much more clear error_log message that pointed to a problem with an .htaccess file inside my images folder on the internal site (that was setup by someone else and that I had no idea existed until the error showed it to me)
That .htaccess file contained directives that allowed my test site to pull images, but not the new domain, so once I added it to the file, all was fixed.
For those that want to know what the directives there were:
SetEnvIfNoCase Referer "^http://test.myservername.com/" locally_linked=1
SetEnvIfNoCase Referer "^http://test.myservername.com$" locally_linked=1
SetEnvIfNoCase Referer "^http://internal.myservername.com/" locally_linked=1
SetEnvIfNoCase Referer "^http://internal.myservername.com$" locally_linked=1
SetEnvIfNoCase Referer "^$" locally_linked=1
<FilesMatch "\.(gif|png|jpe?g)$">
Order Allow,Deny
Allow from env=locally_linked
</FilesMatch>
And by adding the following two lines, it now works:
SetEnvIfNoCase Referer "^http://myservername.com/" locally_linked=1
SetEnvIfNoCase Referer "^http://myservername.com$" locally_linked=1