Can’t access admin: “Not Acceptable”

I found this which might solve your problem:

<IfModule mod_security.c>
SecFilterEngine Off
</IfModule>

This code goes in a file called .htaccess and must be uploaded in /wp-admin directory. This will disable mod_security which is a module in Apache2 activated by default but it can be problematic.

EDIT: @Rarst is right. It is just to see if the problem is here. You’d better not to leave this in /wp-admin directory after testing.