Miguel,
There may be a problem with the nickname on your SmugMug account, containing characters that are no longer allowed for the Site URL setting.
In your SmugMug Account settings page, please remove any non-alphanumeric characters from the Site URL field and reauthorize the SmugMug uploader in Photo Mechanic.
For new accounts, SmugMug won't allow any Site URL with spaces or non-alphanumeric characters, with the exception of a dash. Your Site URL, or nickname, can only have letters, numbers or dashes (-) in it, and can't start with a dash.
HTH,
-Jerry