Windows association does behave oddly sometimes; and more stroppy about doing what you want with more recent versions of Windows than with older ones.
It might be worth trying to create a shortcut to Digital Editions and see if associating it with that works.
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ADE will not open .acsm files in the usual way with ctrl-O.
You can open them if you drag/drop them on ADE.
Obviously you want to associate properly, but may be helpful as a standby.