Today I was running into this problem twice.
The cache icons on the map are not displayed.
When I hover the mouse cursor over the map windows with cache names occurs, but no icons are seen.
In the lower-left corner of my browser there are messages like: loading ..., waiting ..., loading, ... waiting ...
It seems that the problem is triggered by load or network issues.
Restarting the browser, clearing the cache, and resetting the network interface does not help.
But rebooting my computer helps; but this is workaround I don't want to do very often.