You are using a domain/apache/dispatcher in front of this server. Could you check
1) that the configurations do not block you and allow to pass through
2) share relevant dispatcher logs and server logs
3) you have appropriate read/write permissions
Did this all of sudden start to occur and used to work fine? Check all of your OSGi bundles to make sure there is no bundle turned off.
Also - have you installed new packages that may have caused this?
Also - does Classic UI work - this will give us more clues about the potential issue.