I had one of your ideas later, & successfully sent an MMS to myself, received it & opened it. So that seems OK, & the issue seems to be just the one MMS sent on 5/16/2014.
With your information, I looked at /home/user/fmms/mms, & I see two folders: one from the successful MMS iof 3/22/2014, & one from the new MMS of 5/16/2014 which seemed to not work. I entered the folder for the MMS of 5/16/2014 which seemed not to work, & found the photo & message. What is strange is that in the fMMS GUI, it shows the successful 3/22/2014 message as expected: a yellow envelope with the sender's name & no red dot (it's already been downloaded), when it is selected, the picture & message is displayed.
~ $ cat .fmms/mms/NOK5xxxxxxxxxxx/message îäìæàSA½¥ù+4911123456789/TYPE=PLMN ÖΓäú* âüΩ