Your best bet is to empty and break all your safes if you want to keep using them, if they don't open it will be to do with the way they are saved, most likely using the player's name but trying to compare that to the player's UUID.
Yes, exactly what I did. I first updated to 1.7.10 and the problem was indeed fixed, but a fair amount of other stuff was lost/messed up, so I went with the original plan of going all the way back to 1.6.4 and emptying all the Safes and recovering them. Plus tried to recover what else I could.
Surprisingly quick and easy to make backups of the whole .minecraft folder, and just delete and copy/paste a different backup at will, adding the Save folder and the config stuff.
In the end IC2 is in really good shape, now that I've got all my nuclear stuff rescued from the Safes. So joy. I did laugh when I came so very, very close to picking up the fuel rods without a hazmat suit. I almost forgot about that.
Thanks for the advice, Chocohead. Appretiated.
Related, for those interested:-
Nuclear Control lost a fair bit, but considering it's really Nuclear Control 2, I'm fine with that. I can rebuild. I'm just glad NC2 is back.
Unrelated to IC2, but here for completeness :o) :-
It was Railcraft that kinda freaked out, losing a fair number of items (57), weird changes to some stretches of different rail types to plain Vanilla, but not all blocks. And changing a lot of rail types in chests to "tile.railcraft.default.name", but not when in the Personal Inventory, so I recovered a lot of that EDIT: I got that wrong, the rails types revert to "tile.railcraft.default.name" even when they are held in the Personal Inventory, so I guess I'm going to have to create all the rail stuff again. Recovered a lot of Steel, by just converting it into steel blocks, as they didn't go missing, so that was fine. The amount of Crushed Obsidian lost though was... well... I really don't want to talk it right now. I need some time.
:o)
Good news is, the air isn't freaking Steam Ovens, so overall I call it a big win :o)
My thanks to those, past and present, who make Industrial Craft. On top of everything else, kinda insane that a World could remain largely intact over so many Minecraft version updates.