Documentation: https://cosmomidias.com/widgets/Onion-UI-widget.html

Known Issue: Dropbox or Google Drive folders prevent the add-on to manage the cache files.

If you are seeing only the Frames Before Onion Skins, make sure you are on Blender 4.5 firstly (or 4.5.4 on Windows). If the problems persists this should be the Bake Path is set on a Dropbox or Google Drive folder.

How to solve?
Good thing is that you can edit the location where the cache files are going to be manage, and since they are onion skinning cache files (written and destroyed regularly), it is better even don't create them on the Dropbox folder anyways as it does not make sense.

You can keep your .blend file where it is (Dropbox or Google Drive) and point the Bake Path somewhere else (best is a place where you have enough disk space and a quick disk like a solid state drive for best writing/read speeds):

1. Select the Onion-Skinning-Generator object in the Outliner;
2. Go to Modifier's Tab;
3. Point the Bake Path to somewhere ideal that is not ruled by Dropbox, Google Drive, etc. (Note the default was a relative path of your .blend file)

If the problem persists, contact us and send the Console/Terminal outputs spawned during the operation.