When you copy over the old config files from your previous installation to the new one, splunk run a verification if the files are running on a different version and did not start the web, I am not really sure which files splunk uses to check it. The web is a very sensitive part of Splunk and you should be very carefully when you have to restore any config file from previous Splunk versions, because it is very hard to recover the Splunk web from the times I ran into issues.
My suggestion to fix it, please reinstall the last Splunk Enterprise version, because it will be recover the web interface. Be carefully if you have to restore any file from previous installation and avoid to copy the entire folder, because it can cause the issues again. Copy the config files individually and check if Splunk will working properly. This is a manual process and time consuming, thus is necessary to avoid issues when you are copying the old config files to a new Splunk installation.
... View more