Unable to save settings - Garmin Server

Currently receiving frequent complaints about app / watchface settings not being applied or not working at all.
This is what users see when they press Save in the settings list in the ConnectIQ app:

Even on the simulator, I often get error messages:

These usually disappear after a few attempts, but they become annoying over time.
I cannot specify the exact steps to reproduce the problems, they also occur when the settings consist of only a few fields with numbers (to prevent the problem with invalid characters such as \\ in the string fields).