Shaded dashboard issue - really weird? Help!

Our installation of Helical Insight CE has been working really well up until recently when we get a weird issue with our dashboard. When I open a dashboard in the admin account, it works fine almost every time, but when I open a customer’s dashboard, it sometimes (not always) displays a ‘shaded’ screen and I can’t do anything.

It’s strange - sometimes it works, but often it doesn’t. It appears to be having the problem only with my bigger customers with lots of students, because it works every time for the smaller ones. If it is working, if you do a hard-refresh of your browser, that often stops (or sometimes starts) it from working. Cache?

I have included screenshots of a working and not working example - both are exactly the same page, just one showing when it works and the other when it doesn’t. Weird…

Has anyone seen this before or can give me some guidance on why this might be happening?