I somehow missed this thread when it was first posted. As has been talked about above in the thread there is more than one version of the myth, and in some versions (Aesop in particular) it's completely inverted - the box/jar contained all the world's goods, rather than ills, which all fly away when it's opened leaving only hope behind.
There's some interesting options with that reversed version. You could have it be a magical artefact that has effects empowered by spirits of a certain type most or all of which have long since escaped. For example an effect for each of several eidolons or similar emotion spirits linked to some of the positive personality traits (honesty, for example - maybe the cardinal virtues could be used). The stronger the trapped eidolon, the stronger the effect is. Eidolons have a might determined by the value of the personality trait in the person they are linked to, so to get the full effect you would need someone who embodies each trait to a very high level. Perhaps when you have multiple trapped eidolons there is a stronger set of combined effects. There are some very interesting story seeds there (Eidolons! Gotta catch 'em all...)
Alternatively you could actually have the box in this form be a very early divine relic, granted by the dominion to some ancient group. Perhaps it holds some angels who help the owner of the box, but only so long as they have faith and don't try to test God by seeing how the box works (by opening it, or by magical examination etc.). That could be a real dilemma for some magi...
Alternatively alternatively, it could be an infernal item that actually summons demons to plague the owner once it's opened. The demons aren't actually imprisoned in the box, it summons them in response to the owner succumbing to temptation and peeking inside. Maybe it was made by some powerful demon or diabolist to prove a point about human nature.
And if any of the above were true, they could have inspired the legend, which gave rise to a faerie Pandora and the eponymous box as well (which came first, Pandora or the box?). Or maybe more than one similar artefact existed to spawn the legend. Heck, you could even say there was at one point a whole tradition of spell-binders who imprisoned spirits in boxes (hermetic spirit binding has to come from somewhere, right?) so there could be dozens of them of varying levels of power.