When trying to sign online some clients are getting this error message: Content was blocked because it was not signed by a valid security certificate. For more information, see "About Certificate Errors" in Internet Explorer Help. Please could you tell us what causes this issue - is it merely their browser security settings?...When trying to sign online some clients are getting this error message: Content was blocked because it was not signed by a valid security certificate. For more information, see "About Certificate Errors" in Internet Explorer Help. Please could you tell us what causes this issue - is it merely their browser security settings?