It is also my understanding that the loss reported occured in ports. The fact that the Wermacht reports show no big losses at sea is no proof it didn't happen but reduces the probability. The same reports report attacks on convoys on other days.
The same reports also show incidents like mortars exploding during training and killing/wounding some soldiers, or accidental fires that destroy magazines and killed some tens of horses, so it is very probable that any heavy loss of troops will have been reported.
To read more, go to
http://www.wwiidaybyday.com/ and then to the 'Daily reports' section.
These reports came from German books listing original WWII documents. The main purpose of the book is to show all moves and orders of Wermacht divisions but they included very detailed reports about bombings of Germany and occupied countries. I don't remember the book but remember reading part of it in a bookshop some years ago. The price was 100-150 euros, so definetly out of my reach at the time. The books are in chronological order and I read the fall 1940 one. I wondered at the time if the books about the 1943-1945 period were so much detailed to report the bombing incidents in Germany.
On the other hand, the fact that there is no report at this date leaves a chance it was lost. And it is also possible that an empty barge convoy was attacked and so only Kriegsmarine reports have the data.
You can post a query about this convoy on this forum:
http://www.warsailors.com/phorum/list.php?f=1
The title of the forum is 'Norwegian merchant fleet' but it is covering all merchant fleet (and militarized auxiliaires) for WWII (and WWI). Many knowledegeable people use this forum and some of them use original Kriegsmarine KTB so one may have reports about this action. Other are tracking the fate of barges and other small ships and may have in their database losses or damages in this attack.