The first message is due to a missing resource string. Try to load the latest English.hel file.
Are you using some Java applets in your ebook? Or active objects such as Flash?
There is HTML and Javascript. After I compiled a new publication, I could run it once or twice before problem appears - https://i.imgur.com/cTSKOyJ.png
Update. It looks like my content is conflicting with HTMLEXE somehow. My publication is running Powerpoint presentation converted into HTML5 with iSpring. I solved this by executing HTML5 100% on server side with HTMLEXE executing only iframe code.