For some very odd reason, this operation is linked to the Save for Web panel. Recently I've exported some user slices on a different file, using the All User Slices export option and I guess Photoshop made that the default option.
So the fix to this is: create a new empty file and create some user slices
Use the Export for Web option to export them
Make sure you change it from All User Slices to All Slices.
Now Photoshop will interpret that as the default for when artboards are being exported.
Go back to the original file with artboards and try to export it again and it's working (at least it's now working for me)