I was there and it's hard to guess if it was something specific that was said. They were bantering and making fun of each other the whole stream like they usually do, and Uta was joking back just fine. The main issue is that Uta is basically the one keeping contact with everyone the most out of all of them (for collabs and general check ins), so she was constantly the focus of all the topics.
Which means she was also the focus of all the jokes. So it could have been one specific joke or just being the target of it for 2 hours that got to her.
If it was a specific joke it was from Vicky and Miu, Miki was streaming from Japan as she's on vacation and was barely talking during the stream due to set up issues. They were also the ones talking when it happened.
Something similar to this happened a long time when someone in chat made a specific joke that she took really hard. So I personally think that's what happened, as the last things they joked about were about a similar topic.
On the bright side, she will bounce back from it soon, I'm sure she knows they didn't mean to hurt her.
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I hope they redo the collab once she feels better, because they're all really great and deserve a proper anniversary.