Moderator ... please remove the link to the 'README' in ops posting - it discloses server info.
It's been years, but from what I re-call of the setup, server FQDN's were used ... thus, checking internal and external DNS to assure the FQDN variables do resolve to the correct servers.
Also, there could have been certificates generated for the servers involved to exchange or place in the config of the end points. Those certs should have been generated using FQDN rather than IP.
Firewall of entity that's providing the authentication lookups?
Tools to check config ...
https://wiki.shibboleth.net/confluence/display/SHIB2/NativeSPCommandLine
URL is for older (not recommended for use) version of SHIB ... but the commands should still work (would hope).
'spirit of sharing', Ken