Response on feedback assistant request for anyone having the same issue: Remove the management.declarations and management.client-capabilities items from the status subscription configuration. Those items are "implicitly" subscribed to. We will fix the bug that those cause when present, but for now removing them should unblock you.

It appears that just changing the file extension to .txt allows it to be uploaded again. Would still like clarification on what the requirements are for the vendor signed CSR file.