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