To use S-Docs with Salesforce Shield platform encryption enabled, you are required to check a box in the S-Docs Custom Settings list.
Check "Platform Encryption Enabled" in SDocsSettings
From the Setup menu, type "Custom Settings" into the Quick Find bar, then click Custom Settings in the dropdown menu. From there, find the SDocsSettings entry and click Manage.
From there, check the Platform Encryption Enabled field and click Save.