OOS fails after May 2021 update

We are late testing the May 2021 Report Server update, and encountered a problem with Office Online Server (OOS) now failing when trying to view Excel workbooks. We initially received the error message "We are unable to complete your request. Please wait a few minutes and try again." Thinking we needed to restart the OOS service following the Report Server update, we did so. Now when we try to open a workbook we get a gray screen with the message "Connection has been reset". We then tried restarting both the Report Server service and OOS, but the connection error persists. Is there something else we're missing?

The report server is running on a Windows Server 2012 R2 machine, and is rendering both Power BI reports and Paginated reports without issue.