Yes, I can open the .slp pages in RoboHelp. As you suggested, I have examined the Glossary page, in HTML mode, but can find no reference to "glossary1."
Oddly enough, I've discovered that the Glossary tab works properly when my Responsive layout is viewed full screen (desktop), but when the screen size is reduced to that of a tablet or a phone, the Glossary tab behaves as I first described it.