This afternoon has been particularly bad, but these problems have been ongoing for some time.
Many times now I get the message "No such user exists" when I send a PM. I often have to click the back button on the browser and resend several times before WP accepts the PM. Note that the person does exist - and I'm actually sending them a reply to their PM!
I've been told that people also have problems sending me PMs too. Sometimes getting error messages about problems retrieving emoticons.
Sometimes there are error messages with lots of code attached and a message saying "This isn't supposed to happen!"
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I've left WP indefinitely.