I should add that I have been able to respond to comments on another community hosted on a different server, if that makes any difference?
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You can make a post here if you're looking for support with any issues!
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So, a further observation, the people that I can't reply to have the @kbin at the end of their user id. Is that the issue?
Hey! First of all, I don't believe responding to @kbin users is an issue - it was most likely a coincidence.
Regarding the problem itself - there seem to be some spikes in traffic every day during which the server gets briefly overloaded. I would recommend just refreshing the page and reposting the comment if you see the spinner for more than 10 seconds - it's unlikely that posting a comment should take any longer than that even in extreme cases! (Under normal conditions, it should take actually take less than 1 second)