This is very hard to diagnose what happened. You could look at the Radio msag returned by the RFD900 on the plane side/GCS side and see if it stopped being sent. This would mean the radio FW stopped working on that RFD900.
You could also graph the RSSI in the radio message to see if it suddenly dropped? But detecting the cause would be hard, unless it was a psychical issue, i.e.. bad cable connection or other HW failure
I think I found it. I was using an ASUS notebook, designed for Windows 8, 64 bit. But I used Windows 7, 32 bit. It ran fine, but just a few days ago, when I connected to my aircraft, it took about 30 seconds before it started to connect. I thought this was due to version 1.3.10, because it started as I updated.
Yesterday, I decided to upgrade my PC to Windows 7 Ultimate, 64 bit and very much to my surprise, it connected immediately. So the problem must have come from my PC becoming unstable and disconnected by itself.
I will have to confirm this with a next test flight, but I'm quite sure this was the root cause.
Thanks for your reply and taking the time to look at it.