Dropbox has patched the vulnerability and Dropbox Sign is operating properly.

Due to a widespread PHP vulnerability, Dropbox has taken Dropbox Sign offline. Core AdvicePay services, payment processing, Docusign, and Acrobat Sign are operating properly with no issues. If you currently use Dropbox Sign for your processes, you will be unable to send signature requests. AdvicePay will continue to monitor the issue and update this incident.