I had the same issue and found that switching my watch (FR935) off and on again did the trick (I also rebooted my phone, for what it's worth).
Same here configuring some datafields from GCM. 1 out of 4-5 times it works, most of times I try to enter configuration it fails ("There was a problem communicating with your Garmin device"), although connection between the device and mobile seems to be OK. Some time ago (I cannot confirm GCM version) there was no issue configuring same datafields. I have also installed some datafields that can be configured now with no communication issue.
I have the impression that the new mobile "connect iq" app is involved. If that one is uninstalled (and not reinstalled) the setting sync issues do not occur. (Use garmin connect mobile to edit settings )
I had those "There was a problem communicating with your Garmin device." errors long before the new Connect IQ app existed - when I downloaded the app, similar problems (except that the error message was different). After I rebooted my watch, however, both the separate Connect IQ app and the device settings function inside the Garmin Connect app had no problems connecting.
I have just uninstalled Connect IQ app. No difference, GCM allows random access to configuration. Sometimes it enters into configuration, sometimes problem communicating with your Garmin device message is shown
what device and what phone? (iOS, Android, etc)
Edge 1030 (v9.02 beta) + Samsung S10 (Android 9) + GCM (v4.25.1)
Does it stay connect to the edge in general or does the connection keep dropping?