Scenario
We have seen a number of support incidents with intermittent errors when viewing Office documents using Office Web Apps 2013. All Office file types are affected, most often PowerPoint and Word. Often, the same document will display at one time, then throw an error at another. The errors seen by users are varied and are frankly too generic to bother with a comprehensive list. Instead, let's jump into the cause and current resolution.
If you turn up logging to Verbose using the -LogVerbosity "Verbose" parameter in Powershell, you will see in Office Web Apps server ULS logs reference to "ConversionError" and/or "Conversion failed".
CauseAntivirus (AV) network monitoring processes that monitor the *\Program Files\Microsoft Office Web Apps\" folder and all executables (.exe) in each subfolder can potentially interfere with the file conversion functionality. To display documents, each document is "converted" into image files that are cached on the Office Web Apps servers. The process responsible for this conversion is called AppServerHost.exe. Numerous instances of this subprocess are opened and closed by Office Web Apps repeatedly, and AV monitoring software can detect this activity as a threat or otherwise cause these processes to terminate unexpectedly. This in turn leads to a corrupted cached file and the error condition results.
Current Resolution
While attempts to manually/programmatically clear the Office Web Apps cache has had some success, so far the AV programs that cause this behavior must be uninstalled in order to completely alleviate this issue. This vulnerability can be mitigated by any combination of these options: