"AdobeSign Error: Invalid user ID or email provided in x-api-user header" when sending a document for signing

Issue
I am sending a document for signing with Adobe Sign and the following error is displayed:

"AdobeSign Error: Invalid user ID or email provided in x-api-user header"

Cause
The error has been caused by the following:

• Adobe Sign has been federated with Microsoft Azure Identity Management.
• Users for Adobe Sign are controlled by an Identity Management Group.
• Users are NOT part of this group or the provisioning has failed to occur correctly.

Solution
Contact your IT provider and ensure that:

• All required FYI users are part of the Adobe Sign Identity Management Group
• All required users are showing as active users in Adobe Sign and have appropriate rights to send and edit documents in Adobe Sign.