Correction: this doesn't seem to be cas-specific. I've had both cas-icinga and plain icinga pages open in both firefox and chrome for a few hours, and so far i've seen the following failures:

• plain icinga in firefox: Unable to connect (same as first screenshot above)
• cas-icinga in firefox: Unable to connect (same as second screen shot above)
• plain icinga in chrome: Internal Server Error ;)

Here's the network request/response details from the first firefox failure: P11709
It very unhelpfully contains nothing explicit about the failure, maybe it's a simple timeout?

In any case i'm going to close this as Invalid. Sorry for the noise :)