Error on file upload. Error number: 12

Hi

I am using fck ver 2.6.3 with ckfinder.

When I click the insert/edit image icon from the editor > Select the upload tab > browse to my image > Send it to the server

After a few seconds I get

Error on file upload. Error number: 12

However if i use this method

insert/edit image icon from the editor > Browse Server > Upload (ie within ckfinder) it works OK.

I have permissions set correctly on the userfiles folder.

Please advise

Thanks, James