I've finally got around to replacing the old encrypted save system with something that should allow you to transfer save files more easily. This new file format will automatically copy, but not replace, the data from your old files upon booting the game. If you navigate to the "Frebbventure" folder in AppData/local, you might see for example a file named
"frebbventure_save1.sav"
along with one named
"frebb_save_1_newformat."
In case the name doesn't make it obvious enough, the latter is your new, transferable save file. I should note, this is purely a futureproofing feature: if your old file has ALREADY been separated from its original machine, this process will, sadly, not recover it. You'll need to remove it from the folder, or the game will still crash at the file select screen.
Once your new format file has been generated, feel free to remove your old files (but, just in case this all goes belly-up, you should probably back them up.) I plan to eventually introduce cloud saves, and you won't want your outdated saves transferring between machines.