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Same here, have been trying for two weeks on CentOS 7, Centos 6.5 & CentOS 6.6.
In all of them the same issue, ajenti service running Ok, IPTables Ok (for tcp and udp), Port 8000 listening... Ajenti... never get to work on any browser with/without https.

Too bad, I was hopeful since I've heard in the past so many good things about it.
Moving now to CWP (Centos Web Panel).
Still will give the try on ISPConfig as recommended by Idso