I've just been testing a bit and it looks like this beta version does not allow you to enter a custom save path - it MUST be: "ms0:/PSP/GAME/SCUMMVM/SAVE" (Hardcoded)
It works ok for me when i create this path, however i've customised my PSP and am hiding ALL folders (yes even PSPRICK and DOOM) - i hadn't reliased that the custom path setting wasn't implimented for scummvmPSP hence my problems.
I have however come up with a (short term, i'm sure a final realease must be coming soon!) fix...
MY save games (which now WORK correctly) are located here: "ms0:/HIDDEN/SCUMM VM SAVES" - notice the length of this and the original allowed location (26 characters in total).
I have acheived this easily by manually amending the ScummVM Eboot.pbp (the real eboot, not the loader) using a hex editor. (For those that are interested and have hex experience, the offset is 0x00226128 for the m - Don't change it for a longer string though because it hangs on boot if you do)
My saves now work perfectly and i don't get an annoying "Corrupted data" folder in my game list