I respond now even if it's a long time you posted your message but may be it will be useful for others.
I had the same problem after the upgrade to the last version of firmware and I tried a lot of things on NAS and on the router.
The only solution I found is to extract the disk and format with a PC windows, after reinsert on the MS-253 and format with his utility.
Now the bittorrent is working well.
It seems that on every problem we found on this NAS the only solution is to format the hard disk... Funny!