First, let me start by saying you’ve made some beautiful tools. I’m really excited about the potential for integrating these into the product I’m working on.
So I’m working on a prototype for and integration with your viewer, and have been seeing the attached failures frequently. I wanted to make sure it wasn’t my code, so I took the example from https://github.com/groupdocs/groupdocs-ruby/tree/master/examples/viewer and tried uploading using this as well (deployed here for convenience sake: http://groupdocs-ruby-viewer-test.herokuapp.com/upload). I’m still seeing the error at least half of the time I submit a file. I’ve been trying mostly with various PDF and DOC files, and have attached an example PDF on the chance that the files I’m using have something to do with it.