I have the same problem with my TS-459 Pro+.
Now I created a backup of all my data and started the reset process. But that didn't help.
So I searched for the configuration files and found them in /mnt/HDA_ROOT/.config.
Then I've decided for the "Try-and-Error" method -> I renamed the whole .config folder and rebooted my NAS.
The result: It started a installation from scratch (NOTE: It also formats the hard drives!!!).
Now I'll test, if that solution works.
//EDIT: Does not help
For the first few minutes it downloads at full speed. But then the speed decreases to 3 - 5kb/s...
Any other solution?