"The Memory Stick may have changed. Would you like to keep using this Memory Stick?"

[[Unknown]]

Well, if it's happening from scratch for your friends, I would imagine deleting savedata and save states wouldn't help. That said, if you download the latest git build:
https://buildbot.orphis.net/ppsspp/

And then put it in a brand new folder on your desktop, that should be completely clean (it's "portable" in this way.) If it happens right away there, I think it has to be a bug.

A way to test the DST theory would be to change your system time (i.e. in Windows time settings) to a date where DST is not active (future or past.) Then start fresh with no existing save data or save states. If it doesn't happen, that would make it seem pretty sure it's DST related.

-[Unknown]