I have a splunk enterprise out of box installed on win 10.
I can login using localhost no problem, but if I try to connect from a remote machine I get a Red Server Error Message.
This is my web.conf
[settings] enableSplunkWebSSL = 0 startwebserver = 1 SSOMode = permissive trustedIP = 127.0.0.1, 10.70.<MaskedByMod>, 10.50.<MaskedByMod> remoteUser = REMOTE_USER loginBackgroundImageOption = default
Have you checked splunkd_access.log and/or web_access.log ?
Those should provide you more insight into what the issue is.