What was going on
Calling EnvelopesApi::listRecipients with the DocuSign eSign PHP client failed during response deserialization with this fatal error. The SDK referenced the model class \DocuSign\eSign\Model\OfflineAttributes, which existed in v3 of the client but was removed in v4, so any envelope with offline-capable recipients crashed the call. A user patch of commenting out offline_attributes references was not a real fix.
What fixed it
This was a confirmed SDK bug (DCM-3374), fixed in the 5.1.0 release. Upgrade with composer require docusign/esign-client:^5.1.0. The reporter verified the fix on the 5.1.0 release candidate and confirmed listRecipients works. Source: https://github.com/docusign/docusign-esign-php-client/issues/80