I think there may be quite a few possibilities and without knowing all the details on how they are trying to access the envelope it would be difficult to provide a concrete resolution rather I could offer some suggestions....

1) Make sure they aren't forwarding or being forwarded the envelope notification

2) Have the Recipient try using an alternate Browser

3) If they are trying to sign with a mobile device, have them try accessing the document with a desktop or laptop.

4) If they have a DocuSign account (even trial or free) already have them login and locate the envelope to sign or take action.

Past the above suggestions if they still are unable to sign then I would start a DocuSign Support case to attempt to resolve.

Thank you I did suggest those things and they were able to sign the document after all. Vince