That save data is probably never going to be usable again.

AFAIK, the best solution is to disable save encryption in ppsspp.ini. i.e. open the file, search for "EncryptSave" and set the value to "False". Make sure that whenever you update to the latest version or whatever, that that value is always at "false", and if you're worried, make a backup of your save files.

Unfortunately if you've been playing other games e.g. Pure, which doesn't have persistent save encryption problems between versions, you would need to either set the value back to "true" or start all over again with it at false, or the save data for those games will be corrupted as well.