I'm trying to upvote a comment and I get a dialog box with
this comment is not valid for voting or flagging
What's that about?
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Between the time that you started reading the comment to when you flagged or hit the upvote, it has been deleted.
The post you're looking at, to which the comment belongs, is locked.
It happened to me once; the comment was on the question and the question was closed locked. I assumed that was the reason. Other comments on the same question were also not upvote-able.
However, comments on answers to that question were upvote-able, so I don't know what's the idea here.