Hey Simon. I did try pairing my .intentdefintion file down to the bone and removing just a ton of other code to maybe create an Apple bug report. No help. I will add that my watch and iOS widget code is identical save some conditionalization on widgets to include. I never had any issues with old style recommendations I created and even a sample is still shown. Just the screen to configure gives the "unable to load."
Yes, I started migrating to WidgetConfigurationIntent and AppEnum. I was looking for samples to verify configurability on Apple Watch and found a good Medium artcie, but the source code was not directly downloadable.
So I created at:
https://github.com/t9mike/SimpleStatsWidgetConfigExample
But I changed the Water Reminder example to use AppEnum to simplify how the frequency parameter is defined.
This does work on Apple Watch no problem.