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I think your backend account is invalid, because I cant login with it.. anyway I just test in my end and I cant find this issue.. so maybe this issue caused by 3rd party conflict or something else.
I found this issue related with htaccess, have you modified htaccess before ? look at this log :
<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">
<html><head>
<title>406 Not Acceptable</title>
</head><body>
<h1>Not Acceptable</h1>
<p>An appropriate representation of the requested resource /index.php could not be found on this server.</p>
<p>Additionally, a 404 Not Found
error was encountered while trying to use an ErrorDocument to handle the request.</p>
</body></html>
That error (406 Not Acceptable) not come from Jomsocial, that error come from htaccess configuration / server configuration, please you check again..
Please read this information
www.checkupdown.com/status/E406.html
and report to your sysadmin