Just to round things up:
I reproduced the "red cross" in each configuration tab, while taking a log file (the logs are attached and named according to the tabs).
As far as I can see, it is every time the same problem. GCS tries to write the corresponding settings and fails after 3 tries.
Am I right in assuming that in your last comment you are talking about a hardware problem?
In that case I would try to get a refund/replacement...
Thank you again for your help and the great work on LibrePilot (as I said, I am flying it on another quad).