This may give some clues as to why this happens...
I have had the same problems with some of my packages and in my case it is related to a particular charactor in a data element.
<data1>Text blah blah…</data1>
The charactor is question is the 3 small dots before the </data1> known as a "horizontal ellipsis" (news to me).
It was pasted content and I thought it was just 3 fullstops

but it was actually a … although in the xml it appeared as
…. Replacing the … with … in the xml made no diifference.
Removed it and it installed fine. I assume this an occur with other weird and wonderful characters?
And perhaps opening and closing in HTML-Kit corrects the xml for other problem charactors (didn't for me with this one)
I did notice that even though it threw a "
Fatal error: Call to undefined function: ff_query() in \administrator\components\com_facileforms\admin\import.class.php on line 300" the package still installed and could be loaded but all data, elements etc were missing from this line with the "horizontal ellipsis" onward.
This would help trouble shoot where the problem is so upload the package, receive the error, refresh the packages (F5),
export the problem package and open the xml.
You will see that the last full <element>......</element> is the one
before the element that is breaking the upload.
Hope this helps....