This issue was introduced in CC 4.6. It doesn't occur in CC 4.54 and earlier.
It also occurs in CC5 so I can't upgrade.

Steps to reproduce the issue:
1. Create > Transformer > CC3
2. Select a clothing .fbx exported from DAZ
3. CC4 will display a window. Select ADVANCED.
4. When the Asset Import Status Report is displayed, select Maximum Size = 4096
5. CC4 will load the default model wearing the imported clothing item
6. File > Open Project > [my custom CC4 character]
7. Select "No" when asked to save the project
8. Load Project > Replace Character (Keep Outfit) (Note: up until this point, everything is working correctly as normal)
9. CC4 displays "Converting data" and then a window pops up: "Failed to load the file"
10. Press OK and we're returned to CC4 with both the character and the clothing cleared from the scene