I got it to work with KeyChain in Firefox and Brave, but I needed to change something on my side. Normally the requestBroadcast method accepts the operation data in the same format as the SteemJS API does, but this is not the case for the field json_metadata. I guess they changed something in their serializer.
Anyway, after refreshing the page it should finally work now :)
Bingo!
Thank you. This is a great service.