You need to take a look at the ePO server.log and orion.log at this time to get a more detailed description of the error please.
I had this problem when configuring a round-robin AD server. I finally had to configure a specific AD server because they were some problems with cached queries at the round-robin
Hi everybody and thank a lot for your help.
Finally I found a solution. After EPO configuration, It's necessary to rename a DLL on the server to activate "authentication user" in the EPO Menu.
After that all works fine.