While in Participant Center, a participant is trying to take an action such as importing contacts or sending an email, when they get a message:
Error: The requested method is not allowed ... Method must come from a white-listed IP address or provide an auth token.
The participant is getting that message because the participant has lost their login session. It appears that they are still logged in, and they are, but they are partially logged out. The way to resolve this is to have them log out, then back in. This may have happened due to the session timing out which occurs after 20 minutes.
For this reason, we recommend that participants type their message in a text editor, such as MS Word or Notepad, then copy/paste the text in to the email message when they are ready to send it. Please note that session timeouts are industry standard and it happens in all secure applications that stores personal data.