This error may be displayed in the Gmail gadget.
Most often, this is a problem with one particular email. This usually occurs if there is a long string of replies in a single email or if you are attempting to save an email that may be filed in a sub-folder of your Gmail. Insightly looks through all the Gmail folders when it's saving an email, starting at the top, with your main Inbox. If the email you are trying to save is waaaaay down the list (Inbox>subfolder>subfolder or Folder62, for example), Insightly may be having difficulty finding it, which produces the error.
As a workaround you can always forward the email to your Insightly email box instead of saving it with the gadget. This is usually a one-off occurrence.