Apache Reverse Proxy Kerberos

Question

I have a working internal Apache server configured with mod_auth_kerb. I also have an external Apache server that proxies connections to the internal.

When users connect internally with their domain accounts, SSO using kerberose works. When accessing externally, they are prompted for a usersname and password. This setup has worked until we implemented a second domain (aka realm).

Internally, users on either domain can authenticate. But externally, after entering in credentials, the internal server reports

[Mon Sep 24 11:17:17.482681 2018] [auth_kerb:error] [pid 10547:tid 140441496266496] [client 10.200.3.36:49484] failed to verify krb5 credentials: Server not found in Kerberos database, referer: https://server.com/

and the proxy reports

Mon Sep 24 11:17:18.127581 2018] [proxy_http:error] [pid 2539] (70014)End of file found: [client xxx.xxx.xxx.xxx:60926] AH01102: error reading status line from remote server server.com:443, referer: https://server.com/

I know I am missing something obvious.

Here is the config from the proxy

<VirtualHost *:443>
ServerName server.com
SSLEngine On
SSLCertificateFile /etc/pki/tls/certs/server.com.crt
SSLCertificateKeyFile /etc/pki/tls/private/server.com.key
SSLCertificateChainFile /etc/pki/tls/certs/server.com.inter.crt
SSLProxyEngine On
ProxyPass "/" "https://server.com/"
ProxyPassReverse "/" "https://server.com/"
</VirtualHost>

Config from internal server

<VirtualHost *:443>
ServerName server.com
DocumentRoot /var/www/html/wordpress
SSLEngine on
SSLCertificateFile "/etc/pki/tls/certs/server.com.crt"
SSLCertificateKeyFile "/etc/pki/tls/private/server.com.key"
SSLCertificateChainFile /etc/pki/tls/certs/server.com.inter.crt
SSLProtocol +TLSv1.2
SSLCipherSuite RSA:HIGH:-LOW:-RC4

<IF "'%{REMOTE_ADDR}' != '10.200.3.35' && '${REQUEST_URI}' != '/wp-admin'">
AuthType Kerberos
AuthName "Kerberos authenticated intranet"
KrbAuthRealms REALM1 REAM2
KrbServiceName Any
Krb5KeyTab /etc/httpd/merged.keytab
KrbMethodNegotiate On
KrbMethodK5Passwd On
require valid-user
</IF>

<Directory "/var/www/html/wordpress">
AllowOverride All
</Directory>
</VirtualHost>

Keytab sample

slot KVNO Principal
---- ---- ---------------------------------------------------------------------
   1    1 HTTP/server.com@REALM1
   2    1 HTTP/server.com@REALM1
   3    1 HTTP/server.com@REALM1
   4    1 HTTP/server.com@REALM1
   5    1 HTTP/server.com@REALM1
   6    1          HTTP/server.com@REALM2
   7    1          HTTP/server.com@REALM2
   8    1          HTTP/server.com@REALM2
   9    1          HTTP/server.com@REALM2
  10    1          HTTP/server.com@REALM2