Error checking backup file. moodle.xml not found at root level of zip file.

This has now occured in all versions I try on this server. Unzip does work; it looks like the temp directories are being built properly but nothing is ever unpacked in them. I end up with a bunch of orphan directories with the original backup file in them. I have also confirmed that the file moodle.xml is at the root of the zip and is of the proper format. As a note, during debugging we took a small existing site on the server, backed it up then went to do an immediate restore and experienced the same issue. At this point we are stumped; the site where the original file was made was able to doa successful restore without issue. It seems to be related to the behavior of the zip program, but there are no detailed logs I can follow nor any debugging I can turn on to see exactly what is happening. There is reference to a "SILENT_RESTORE" flag in the code of restorelib.php but no reference on how/where to set it up.

MJK