Hi,

Where do you see this error? In the logs, or in the console?

Looks like application issue; another possible cause: are you connecting directly or via a proxy/reverse-proxy ?

I can't promise anything, but I'll try to figure something out.

This is extremely strange. Looks like something is hanging on server side. Please run ajenti-panel with -v switch, reach the "gears" stage, and then press Ctrl-<backslash> in the terminal. This will output (among other) a stack trace. Please post it.
(Ctrl-D Ctrl-C to stop panel later).

What browser are you using? Is this issue browser-dependent?

Please also check pstree output to see whether there are hanging child processes when the gears appear.

Please stop the service, then run ajenti-panel in foreground and look for errors.

This means Ajenti service is running in background: 
$ service ajenti stop

This looks fine. Maybe you could instead run ajenti in foreground on server side and check for any erroneous output?

$ ajenti-panel -v

Please check the browser console