Office Online Server "An item with the same key has already been added" No FileSourceId found for servername.

I was experiencing errors rendering Office Apps from my SharePoint farm after attaching a new Office Online Server to he farm. I saw the following error in the browser:

Microsoft Word Web App Sorry, there was a problem and we can't open this document.

After some troubleshooting, I found this error in the ULS logs of the Office Online Server (Office Web Apps):
No FileSourceId found for servername. (servername was the name of my SharePoint URL)

I solved that by running the PowerShell cmdlet: New-OfficeWebAppsHost -Domain servername

This should have solved my problem but I accidentally entered my domain twice (corp.domain.local)

I now received this error in my event log on the Office Online Server ID 1309

Process information:

Process ID: 9999

Process name: w3wp.exe


Exception information:

Exception type: ArgumentException

Exception message: An item with the same key has already been added.

I had to run Remove-OfficeWebAppsHost -Domain corp.domain.local

It removed the extra entry and everything is working great now.


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