Got this reply from Garmin today:
"Thank you for contacting Garmin International.
I will be happy to assist you with that. In the last few weeks we became aware that this was happening, the engineers are working on it and it should begin to work this week, I see that you sent this to us on Friday the 2nd of May, please keep us posted if you do not see any improvements on Garmin Connect by the end of the week. Also keep your workouts on the device as we can reload them one at a time after deleting the activities from Garmin Connect once it is working properly again.
Thank you for your patience with this issue."