I am not sure what you mean. Do you mean to test each cell of the first string individually? Anyway, I did a capacity test of the pack as a whole as described above: the BMS cuts off after much less than the nominal capacity of 40 Ah because the first string has low voltage. This way I was able to locate the problem and therefore, I plan to open the pack and locate the cell that causes the first string to leak energy. I just hope that I don't have to isolate each and every cell of that first string.
This morning, I found the first string still at 3.33V. The others were at 3.6V. To me it is obvious that the first string has a short cell that slowly brings down the voltage and the capacity of the whole pack. I guess that after I replace that one cell, the problem is solved.
Anyway, I will ask the chinese seller to send me a few cells as a compensation.
I doubt you have a shorted cell. It's much, much, more likely to just be crappy quality cells, probably with a few cells in the pack that are way under capacity, or maybe even open circuit. A voltage test isn't going to tell you much, if anything, about the state of charge of each cell group. You need to do some testing under load, plus ideally doing a capacity test on each cell group. My guess is that you'll find that the pack capacity isn't 40 Ah (it's pretty common for the vendors of these things to just openly lie about capacity) and you'll probably also find either some dead (open circuit) cells or some crappy internal connections (probably both) if you take the pack apart.
Unfortunately there are a LOT of nasty cylindrical cell packs around, more often than not of poor internal build quality. Dogman was spot on earlier with the comment earlier about not recommending those packs.
Good luck with getting recompense from the vendor, many of these Chinese ebay sellers are just buying up unknown quality surplus cells in the big electronic street markets in Guangzhou or Shenzen, assembling them into packs, selling them on ebay until they get caught out, then starting the process again with a new ebay ID. There are a few Chinese based battery sellers who have built a solid reputation, people like Li Ping and Cellman, for example, but there are also hoards of rip-offs to catch the unwary. The unscrupulous vendors rely on the high cost of postage and the absence of any consumer protection laws for international sales, so can get away with what we'd call fraud. I doubt the photos on their ebay advert have anything to do with their batteries, either, they were probably just copied from a reputable manufacturers site.
Please ask questions on the forum, rather than by PM, as it helps others and you'll get a better range of answers.