MarcEdit 5.9 Hotfix
Sorry folks – apparently a hotfix Microsoft pushed out caused the setup tool I used to corrupt when working with 64-bit systems. I’ve removed the hotfix and rebuilt both version (32 and 64) of the program and it looks like things are in good shape now.
If you tried updating today and had trouble, give it another go. If you updated and didn’t have trouble, you should be fine (though you’ll be prompted to update again).
Sorry about that folks.