Basically this herbal "tea" that they were all drinking whole way to this city was poisoned.
Even after they all agreed to not drink it, claiming that director was behaving strangely, they still drank what he gave them. And FeMC was also quite strange that she didn't give them some of her vitality pills or some other meds. Yeah, she knows that being overly reliant on meds makes your body weaker, but if your whole team feels weakened without any reason, you give them meds until you find the source of their illness.
Stupid AF, but you somehow had to knock out super-elite tier magic knight and mage.