Original reporting was that the Russians poisoned Ukrainian negotiators, along with a Russian oligarch trying to facilitate those negotiations.
Later reports seem to indicate the illnesses were due to unspecified environmental factors.
”Ukrainian negotiators were unwilling to confirm the incident. Rustem Umerov, one member of the Ukrainian team, told Reuters that people should be wary of “unverified information.” Another negotiator, Mykhailo Podolyak, said “there is a lot of speculation, various conspiracy theories.”
Toronto Sun
This piece also reports on the confusion among explanations:
Sky News
I need to break away again, and this is a developing story that could change at any time, so view it as a place to start, not definitive.