I've got an ATIK 420 (imager) and ZWO ASI120MM-S (guider) on a remote Raspberry Pi running Ubuntu Mate and an AMD64 Running Ubuntu 16.04 as client running EKOS
Intermittently, the client can not down load images from either camera. When Preview is clicked the exposure counts down then "Downloading..." is displayed but never completes.

This problem has been driving me crazy for some time so I decided to tackle it methodically and I think I have come closer to isolated the cause.

Running PHD2 on the RPi and connecting to each camera through Indi I could ascertain that the cameras and drivers were working ok.
By isolating the network and running over cable I could rule out problems with the wifi network.
Setup with minimal equipment and cables to rule out earth loops.

Finally made the following discovery:
I had set up EKOS to use PHD2 guiding on the RPi.
If I reverted to the internal guider then the images would download and display.
If I enabled PHD2 on the client the images would not download
If I enabled PHD2 on the RPi the images would not download.

Going in further I started PHD2 on the RPi and started looping.
If I then connected from EKOS I could capture images from both cameras. In fact the images from the ZWO started coming through automatically - even though I ha not yet connected to PHD2.
But if I then connected to PHD2 things started to go awry again. And from there I stated to see some inconsistent behavior depending on when I started looping on PHD2

So anyway, it looks like the downloading issue is not to do with the camera driver but rather the PHD2 interaction.

Thanks for detailed report, I'll investigate it.

Just a wild, slightly educated, guess; but is there a timeout on the PHD2 socket so it doesn't loop forever when there is no message? That would cause problems if PHD2 isn't in its capture loop; but when it is it is sending a message every couple of seconds so doesn't lock things up