Scenario: Lightroom CC 2015.12, running on Windows 10 v1703, all completely up to date.
I have an existing folder hierarchy, and I've added three images into an existing folder that LR knows about.
I right click the folder in the Folders panel of the Library module, and choose "Synchronize Folder..."
Lightroom correctly identifies that there are three new images to be added:
Notice I have selected "Show import dialog before importing." I want this to be able to add keywords, metadata presets, etc.
However, when I choose "Synchronize," I get this blank import dialog. This is disturbingly similar to the very long standing import dialog bug where the thumbnails don't show up until you choose a "Destination." However, in the "Synchronize Folder" scenario, there is no destination tab on the right, because the destination is fixed and we are in "Add" mode:
Notice that any forward progress is completely halted - the "Import" button is greyed out, and there is no possible action other than to cancel the operation.
If, during the sync process, I De-Select the "Show Import Dialog" option, the images are imported correctly (but without the ability for me to pick and choose, add metadata and keywords, etc):