what did you use? FAR?
Changing the text of pref_store.txt changes the location in which sms messages are stored (i think).
The original contents of pref_store is /smsdata which you changed to /tpa.
You cant save any messages? Does that mean you cant "save" a message (as in store it in the Saved Messages part) or does that mean that no recieved message is saved in the inbox?
Also you could try by making these folders using FAR:
/tpa/messaging
/tpa/messaging/sms
/tpa/messaging/sms/msg
/tpa/messaging/sms/FFFFFFFFFFFFFFFFFFFF
/tpa/messaging/sms/FFFFFFFFFFFFFFFFFFFF/pending
/tpa/messaging/sms/FFFFFFFFFFFFFFFFFFFF/sm
/tpa/messaging/sms/FFFFFFFFFFFFFFFFFFFF/sr
/tpa/system/messaging/sms
They are present in your /smsdata and are required to classify messages into their categories.
hope that helped :D
EDIT: Apparently the whole procedure does not work for a lot of phones so I'd recommend just undoing everything by rewriting /smsdata in the txt file.