I have the exact same issue. 

One of our computers shows this message.  

We have 3 companies, so we have 3 different company files that we use.  

The message pops up after a period of time.  It is not related to the company file.  

I also tested with a totally clean file that had no information in it.  Still happened. 

Not sure how the scam people got that on the computer.  

I did some digging around in Task Manger and noticed a second QB running process and I right clicked it and then clicked on Open File Location. It took me to the folder C:\Users\Public\Documents\Intuit. Inside there, there was the normal QuickBooks folder that should be there, but there were also 8 files in this folder that were not on any other computers at the client's office (https://imgur.com/a/g9ICcbc). I double clicked the (err.html) and sure enough it was the popup. I deleted all files in the folder and then restarted the computer. So far no popup, but I will have to see if it comes back at the end of the day.

I have no idea how the files got there (user swears he did not download anything). I will keep monitoring this computer to verify that the issue is fixed.