Are the saves no longer PSP-compatible?

[thedicemaster]

it may be related to save encryption.
i don't know the default settings for ppsspp, but it can cause problems when swapping saves between ppsspp and a real psp.

i used to have problems with ppsspp incorrectly loading encrypted psp saves for certain games, it may be possible in your case that ppsspp writes unencrypted saves and that the psp game can't handle this(if you have a cfw, some may allow for saving/loading unencrypted saves using a plugin)

some games also have protected saves(djmax series for example) which will see any save from a different device as corrupt/blank.