Ok thanks for the info mdbeck.
Since you didn't have to solder directly to the terminals of the cells then heat damage to the cells couldn't be a factor. So yeah, they might be rubbish cells or at least unable to stand up to your charger.
Can you tell us roughly how many cycles it took before the pack was degraded? Also, what was the failure mode. Did you notice it losing all its charge from one day to the next, or was a drastically lower run time the first thing you noticed?
BTW. There's one other thing that could have caused an issue with the rebuild. All of these packs have a thermistor stuck somewhere there between the cells. It's used by the charger to tell if the pack is overheating and halt or modify the charge cycle if so. The thermistor is a little two wire device, usually with one wire connected to the negative and one connected to a third terminal on the pack.
Different thermistor types can vary quite a lot in terms of their exact characteristics, and the correct one to use is the one that came with the pack you're rebuilding,
NOT the one from the new pack. This is an easy mistake to make, people often think that the thermistor should "go with" the batteries but it's not correct. The thermistor should "go with" the charger.
One final thing. When repacking try to get the thermistor buried right in between cells if you can, it gives a better temperature indication.