Office Online Server: X-WOPI-ServerError "Verifying signature failed"

Problem

After migrating from old OOS (WAC) server to new one, integration between SharePoint and Office Online Server stopped working, and no documents could be opened in browser.

Looking at the network traffic using WireShark, you see X-WOPI-ServerError "Verifying signature failed".

Solution

1. Ensure HTTPS (TCP 443) flows from SP servers to OOS
2. Run Update-SPWOPIProofKey -ServerName YOUR_OOS_URL

This will update the required keys in SharePoint and documents can again be opened in browser.