As I said in the OP, we have tried deactivated all plugins. That was before the client fixed the js error, so I have just tried it again, and no change. Change of theme does not solve either.
I have had another site where js minification that caused this, but we have no minification on, and I’ve turned caching off. Console is not showing any errors on the page.
It does “look” like the plugin is deactivated. I’ve checked the db, and SLP is in the activated plugins list.
What else could be making the plugin not function?