… on the SharePoint 2010 with german system language of my customer the message text was “Fehler bei PowerPoint Web App. Wiederholen Sie den Vorgang.”

In the SharePoint log I found this lines:

First: “Could not find default ServiceProxy or it does not implement IWacServiceApplicationProxy”

Second: “Unhandled exception caught when executing web method handler: System.NullReferenceException”

The Solution:

There was a missing setting for the Service Application Default Proxy Group

Go to: Central Administration –> Application Management –> Configure service application association  ==> click the link (e.g. “default”) in column “Application Proxy Group” of your web application.

You should see something like that:

Check the missing proxy. Click OK. That’s it…