How exactly does this field  do what it does? Without any further
information, I'd guess that the field is -- at some level -- dependent on
another and starts complaining when it can't find what it's expecting.

That's rather vague, but it's all I've got based on the current level of
exposition.

I ran across this custom  field a couple months ago. It implements cascading
dropdown lists using custom field  types. You can find more info here:

datacogs.com/datablogs/archive/2007/08/26/641.aspx

You are going to need use the debugger to find out exactly what is happening.

This is what I suspect: Your drop-down requires a URL to determine the site
containing the list  used in a dropdown. When you deploy to another site
collection (or web application), the URL is no longer valid, and therefore
cannot find the list for the dropdown.

Assuming the above is the case, you will have to modify the code to handle
the "incorrect" URL.

You must debug the code to determine if my assumptions are correct.

I originally faced the url issue when I did a restore to another farm
and we had to fix that issue. However, within a farm, the drop down
can point to any site  collection, sub site, etc and will work  fine so
I dont believe it is a url issue.

I think Paul mentioned the datacogs drop down. That is what we are
using after making a few mods to the source code they supplied. The
first error  is thte get context method and we are wondering what the
import is trying to do to invoke this method?

When you deploy from one site  collection to another, the assumed URL defined in
the CFT is no longer valid. The code must be able to handle that.

Whether or not the URL is the issue, you'll still need to debug the CFT to
verify. Then, you'll know for sure. I used the Datacogs parent/child dropdown
which worked incredibly 99% of the time. I had to make some minor coding
changes to deal with deployment issues.

We have fixed the problem. Without going to deep, the get_context
was trying to get a hashcode from the current session, however during
the import  there is not a current session. It is a hack, but we
checked for null and assigned the variable a fixed value if it was
null which only seems to happen during import. Fixed!!!!

We also had to make a few tweaks as well. We ended creating a new
field that requires only 1 custom  field, not parent and child, that
change be filtered N deep and can also use a view to help futher
limit the selection results. The next step is how to fix existing
list to change out that custom field....but that is another day.

I don't think you will be able to Export/Import it after the custom  Field has
been configured to point at specific parent lists. The problem is that when you
Export you lose a lot of the GUID data stored in the site. Then when you import
on a new site  that GUID data is normally replaced automatically. I don't think
the GUIDs for the parent and child lists are coming across, but the column still
thinks it has already been configured. There are things that can't be Exported
and I suspect this custom column is one of those things.