Looking through the logs I can see one miner submitting mostly rejects - all below target level, suggesting dodgy firmware on his hardware, but I don't see anything else untoward. That might be enough on its own. I suggest each individual user check their own stats and see if their hashrate at the pool reflects their hardware - ckpool reports hashrate based ONLY on accepted shares.
Thank you for the info ck. That's good to know. So if the difficulty is too low on a particular miner, even one with a relatively low hash rate, it's possible to rack up a large amount of rejected shares. That's good to know!