I’m not sure if my problem was exactly the same as the other posters here but during my searches I kept coming across this thread which doesn’t really have a solution, so I’m posting this in the hope that it might help someone else out. I solved mine simply by recreating the asset directory structure within the subdirectory.
After literally hours of searching and despair I decided to actually read the notes for "File base path" in the manager, and it says:
The File Browser directory must contain the subfolders images, files, flash and media in order for the file browser to function correctly.
So now my folder structure looks something like:
Now my manager users can upload and insert images and only ever see what’s in their own folder and now TinyMCE is happy, although I also had to make sure the maximum upload size was set large enough.
I’m no PHP guru, so if any MODx pros here can see a downside (esp security wise) to this method could you PLEASE TELL ME.