Don’t worry there is a workaround for this problem! The solution would be to convert your joints to bones, export the bone based character and import is back again.
But wait! You still can’t convert multi-mesh joint based characters to bone based ones because that part of the program doesen’t work properly either. The conversion works on only one mesh. But don’t worry! You just need to collapse your character to a single mesh! …and…oh no…my god…if you connect the already weighted meshes you’ll lose the weights. Because no-one really implemented such a feature. They must have thought: why would anyone be so dumb to collapse an already weighted character?
But again: don’t worry! Because you can repaint your weights…in Claude Bonet. That tool is such fun:) By the way it has no auto weight function. Which is its smallest problem because I think noone understands how it works, since you can paint one vertex to 2 bones with a 100% weight applied to each bone. But you really can’t blame Maxon for not repairing Claude Bonet. Again they must have thought: who on earth would be so dumb to use CB instead of the new weight tool?
So, there is an actual workaround. And it really works:)

Hope this is helpful.