I must firstly confess, I don't use mashups a lot! However in August 2019 I created a mashup and it worked perfectly. I am trying to create a very simple mashup today but cannot get the mashup to show, when launching the html file..
'Help' mentioned things like restarting Sense, which I did, but it makes no difference. Could there be a issue with Qlik Sense Desktop February 2020: