Trying to open an *.mcworld file give "Level import failed"

My son and I have been playing rather extensively on a world since the first aquatic update.  We have (had?) a lot of stuff.  When I saw the second aquatic update was available, I exported our world to back it up, upgraded, and we started to play the newest (v1.5.0).  Shortly, it crashed, and caused my son to lose all of his items, so I thought "Easy... I have a backup!".  However, trying to start again with the backed-up *.mcworld file won't work at all.  Clicking "Open" from file explorer, or using the import button having launched MC normally both give the same result: almost instantaneously after "Level import started", it results in "Level import failed" with no further information.  This is quite confusing.  I can't even import a level exported by the same, new version.