i have the same error, not exactly the same but...
all the notifications emails to users have the subject and the body empty, the email are being sent by the correct user at the correct domain.
I have make some translations in english.php file, but i test with the original file and the result are the same.
My version of photopost classified is 2,6 (i have upgraded from 2.5)
thanks in advance and sorry for my "indian" english