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?
We are sorry to hear that you have such issue. Looks like that this error is related to the browser security settings. Could you please share with us URL which you use for embedding GroupDocs.Signature App and your IE browser version that we can test it and provide solution for the issue.