@peterleichen
fmms version is 1.3.6-1

Sorry, the exact error is "Failed to retrieve" and is inside the file "message" of mms folder.
But... images and text_0.txt are there and perfectly readable.
The "headers" file contains:
Transaction-Id 99905
MMS-Version 1.3
Message-Type m-retrieve-conf
Date Tue Jun 30 17:43:00 2015
Message-ID mS5R8-ZkUZf1DbTFkZvF00
Content-Type ('text/plain', {'Charset': 'utf-8'})

I don't receive mms so often, but all the mms received in the last 2 months behave in this way.

Do you think that this (maybe decode) error could be cross checked verifying the presence of not empty attachments/parts in the mms folder? In this way we could at least see the attachments succesfully decoded... something is better than nothing..
A message like "something went wrong but part of the message has been received. Do you want to retry or open the parts already decoded?"