all this recent 3dsimed issues. Try upgrading to 3.2
http://sim-garage.co.uk/software-release/3dsimed3-2a/
I have no issues really, and it is my main tool. You do need to understand its oddities and quirks, as I have very few issues using it.
note: I have quite bad consistency with unpacking .kn5 --> .fbx with CM
Some 3DSimed Tips (for tracks)...
if you move/delete a previous collision mesh, you must export to FBX and run kseditor
you can adjust many shaders and add AC_ objects direct in kn5 (and export to .kn5)
there are some workflow caveats to loosely observe depending on original source
1. if you have only one object from an original FBX, try direct conversion to .obj in 3Dsimed, and load that into a new 3Dsimed instance
2. if the above does not work... "tools", "split objects by material", save as FBX or .obj
My "typical" simple process for putting a track in AC from raw FBX:
1. import .fbx to 3dSimed, convert to .obj
2. perform edits, mesh naming and check shaders/textures
3. export to .fbx
4. open fbx in kseditor
5. save as .kn5
6. open .kn5 in 3dSimed, place AC_PIT0, save as .kn5
7. run in Assetto.
note: you DO NOT NEED any extra files or folders other than your track .kn5 for it to load and test drive from AC_PIT_0. An empty AI folder will be created on first run.