Hi, I've unfortunately nothing in particular, at least for the given clues, but maybe related, could it be that the invitation email content still shows "Searching for an existing appointment..." above the buttons?
That is not the issue. It finishes searching and correctly shows whether there is a conflict.
It also depends where you receive the message, because some folders are treated as special folders, like Sent and Outbox, where is done no searching for existing appointment. I do not think it's it, but just in case.
I have successfully responded to invitations from others in the same folder.
Otherwise, if it's truly sender specific, do you have enabled search for sender photograph, which might not block the itip mail processing, but who knows. The option can be found in Edit->Preferences->Mail Preferences->Headers tab.
I do not have this setting enabled. There is one clue I omitted from my initial message. The user who sends me invites that I cannot respond to always sends two ICS attachments per invite, both in the same message. They are identical except that one has a newline at the end of the file and the other does not. I asked him about this and he said he doesn't do it intentionally.
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