Under the "Device" widget, there's a message, "You have no device on Garmin Connect".
However, the device (810 in this instance) is connected on Garmin Connect - therefore I couldn't upload activities.
I switched back to "Classic view" and was able to upload by clicking on
Suggestion - put the "search for devices" button into the Modern View.
Also, in general switching to see detailed activities is much slower - the Modern View might be more appealing but needs the at least the same speed as the Classic.