Hi Sandi,
First up, I highly doubt that this is related to the validity of the Reader Extension credential, nor is that something you even need to worry about.
If one of the signatures is in an unknown state (the question mark icon) then there is a prompt to warn the next signer that they are signing a file with signature problems, but it doesn't prevent signing from occurring. Let's get some background information to help figure this out.
First up, do you know what application is being used (e.g. Acrobat or Adobe Reader)? Next, what is the version of the application? Finally, this message that users are getting, can you tell me what it says verbatim?
Thanks,
Steve