You might try this trick to get it working. I noticed on a few occasions that it helped.:
1) Go to your MYDOCS/APPLINKS/3DCOAT/EXCHANGE folder and delete or move to a separate location, the BLENDER folder.
For some strange reason, the contents of that folder mess up the Applink once in a while.
2) Test a simple export of the default Cube in Blender, to 3DCoat and back
Please add that io_coat3D folder into that location. See if that fixes the problem.
EDIT: I can understand that if you save blender in your AppData folder this could cause problems.
Noticed that udim workflow wasen't working. Now this update should fix this issue. So right now you can take one or more meshes into 3d-coat with one click as long they share same udim set. For example if you have cube 1001 and sphere 1001 that dosent still work. Here is quick demo how this feature works. Most importan thing to remember:
1) Turn on "Import tiles as uv-sets" from import window
2) Turn off "Export uv-sets to different folder" from export window
This uptate will already included in daily builds but here is zip file too. io_coat3D.zip