After upgrading to v3.5 we noticed that newly uploaded PPT files did not show using the office viewer plugin. The preview stays black and keeps loading. On the server side I could see that the slide images had been created.
We figured out that the uploaded files have spaces in the filename. I believe the previous version (v3.1.1 for us) automatically removed spaces from uploaded file names. We were able to work around this issue be re-uploading the files without spaces in the filename and then the plugin worked again.