If the Web server detects that the data it wants to return is not acceptable to the client, it returns a header containing the 406 error code.
This is propably from Firewall > Module Security
if your script is
then you can bypass the mod secutity in a folder/domain bur its in your oun risk.
You can put following code in .htaccess file if you want to disable it
SecFilterEngine Off
SecFilterScanPOST Off