Keep loading after clicked "Send Out for Signature" in Salesforce (classic view)

Hi there~

Some of our users are reporting issues with generating Adobe Sign agreements in Salesforce.  When they are in a Salesforce Opportunity and click the "Send Out for Signature" button (in the SF Classic interface), the Adobe Sign window that pops up just shows the "loading" icon and never actually progresses to the next screen. Does anyone know such issues? Thanks.

Keith