Sometimes when I resume the GCM app, fields such as calories, steps, and sleep don't load correctly until I restart the app, but the error message seems to be specifically referring to the "weight" screen. That screen hasn't loaded any information since I first started seeing the error message in the orange bar that states some data couldn't be loaded (see attached image).
The weight entered in my linked MyFitnessPal account does sync to Garmin Connect on the web. I just can't view that information from the mobile app on Windows. I even installed the UWP app on my desktop computer to make sure it wasn't a data connection issue with my phone, but the app on my desktop gives the same error message.
I have the same problem with "weight" screen on Garmin Connect Mobile for Windows 10 Phone (Lumia 950) and Windows 10 PC (Desktop/UWP)!