In an on premises SharePoint farm, Word, Excel, and PowerPoint files open for viewing and light editing in the browser through a separate server role. Microsoft called this Office Web Apps Server in the SharePoint 2013 era, and renamed it Office Online Server starting with SharePoint 2016, though the underlying mechanics that connect it to SharePoint have stayed largely consistent. When it stops working, documents that should preview or edit in the browser either fail outright or silently fall back to opening the desktop application instead, and the cause is usually the same thing: a trust relationship between SharePoint and the Office Online Server farm has gone stale.
The core fix: resync the WOPI proof keys
SharePoint and Office Online Server authenticate each other using WOPI proof keys, which rotate periodically. A background timer job is supposed to keep both sides in sync automatically. When that timer job fails to run on schedule, the keys drift out of sync and the connection breaks. In the SharePoint Management Shell, run:
Update-SPWOPIProofKey
This forces an immediate refresh of the proof key SharePoint uses to trust the WOPI application, which is Office Online Server in this context, and resolves the issue in most cases without needing to touch anything else.
If the problem keeps coming back
A one time fix that recurs points to the underlying timer job schedule itself. You can adjust how often it runs with:
Set-SPTimerJob "job-rerun-discovery" -Schedule "<your desired schedule>"
In some environments, the SharePoint Timer service itself needs to be stopped and restarted on the affected server before the new schedule actually takes effect. If Update-SPWOPIProofKey does not resolve things on its own, checking that the SPTimer service is running normally is worth doing next.
If your farm is on SharePoint 2016 or 2019, keep in mind extended support for both ended this month, which makes Office Online Server issues like this a reasonable trigger to revisit whether staying on premises is still the right call. Our cloud migration services and Microsoft 365 team can walk through the options with you. Get in touch if a document viewing issue has turned into a bigger question about your SharePoint environment's future.