1.Run this PowerShell Get-OfficeWebAppsMachine
to return the health status of your Office Online Server.
2.Remove all WOPI bindings from SharePoint farm then recreate with PowerShell command.
Remove-SPWOPIBinding -All:$true
New-SPWOPIBinding -ServerName <WacServerName> -AllowHTTP
3.Find clues in the ULS logs.
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