I was having a heck of a time trying to import a custom object to use in my terrain. I came to believe my problem was just total poly count as each sub component would import by itself, but not when everything was combined. Now, I am going to go back and reduce my poly count. I got a lot of fat to trim, however, the weird thing is I was under the poly count of other standard objects I had exported to study. So that didn't seem fair!
As a test, I exported shape 1650c. All I did was rename the .ac file to 1650cx and then try and re-convert it back using AHObEdit. It crashes everytime. Sadly it doesn't produce a dmp file. I suppose, the .ac conversion could have a stricter limit than what method you originally created the standard objects with, but it would see odd to a user that you couldn't re-import the exact thing you just exported.
Attached in the exported file to try importing.