The four people trying have 2 iPhone 2 Android plus I am using an iPad and Samsung tablet.Have you tried Android or iOS ? I never got any popup on my computers.
Task 1 is triage. Task 2 I want to hire someone get a selenium monitoring system setup. Performance + monitoring for stuff like this. Maybe setup a EU and Asian VPS to help the monitoring.Could be difficult to recreate. I suspect DNS poisoning and that would be a location/ISP specific problem. You won't replicate it in CA if the problem is in the EU.
Ads served are location specific as well, which is equally problematic.