In my case it was solved by re-installing the OWA/WAC farm with the render cache on the C drive, which I had previously installed on the D drive. It didn't have to be in the default location but it didn't like being moved off C.
In my case it was solved by re-installing the OWA/WAC farm with the render cache on the C drive, which I had previously installed on the D drive. It didn't have to be in the default location but it didn't like being moved off C.