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.