Really Ugly Workaround
I'm having the same problem. I think the cause is that the file is missing its end. (I have a file that claimed to be 800 megabytes, but I only downloaded 500, even though I got no error message).
I did find that I could get most of the file from the command line. When opening the file, the Archive Utility will expand everything that IS present, then when it discovers the file is not complete, it DELETES it again. :-(
To save the partial file is ugly, and requires both the command line, and Apple Archive Utility
open a terminal windows (Find terminal under applications)
Type the following (lines starting with # are directions, don't type them!)
cd /tmp
# Type open, and the full path and name of your file
# like open /Users/john/Desktop/myfile.zip
open path-to-your-file/filename.zip
# BEFORE THE ARCHIVE UTILITY FINISHES:
# Click on the terminal window and type
ls -al
# Look for a file named with a name random name (like .BAH2NMxl),
# it should have the current date and time on it. This is a temporary file
# that is created while unarchiving the file.
# type cd (the name of the temporary file)
cd TEMPFILENAMEGOESHERE
# this will keep archiver from deleting the parts of the file that it DOES have
# When the archiver finishes, type
open .
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You now have the pieces of the file that were in the archive.
I don't promise this will work for you, or that this is the best method, but it got me most of what I needed.