I can add an answer but I can't comment on any other answer. I havea reputation of over 10K and I have been able to comment for over a year, so wht is happening today to make the site decide I can't comment?
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Have you tried clearing your browser cache?
Sometimes browsers / proxies inappropriately cache outdated versions of the files we serve and this causes problems.
Are you still shown as being logged in? Occasionally when I've suddenly lost the ability to do stuff, it's because the system has forgotten me for some reason - logging in clears everything up.