It just happened, not a few minutes ago. 12:57 pm local time (UTC -4). It was there and gone before I could get a screen-cap, but it looked just all the other error messages the software gives. Black box highlighted with red. This is not the actual image, but another example I found online. Same message, though.
The only error I can see in the server log at that time is an anonymiser error. But that happens like every 4 minutes, so I doubt that is related. Based on the message it sounds like maybe something weird happened to your credentials. You were logged in and sent a request to the server, something happened before the server received and suddenly it had invalid credentials appear in the middle of something. A refresh would cause the request to be sent again, with correct credentials. That's just a guess, though.
I know this is an old thread, but I wanted to comment for Wrey and members of the admin team so we can recognize and mitigate these types of errors in the future. This is most likely an error triggered server-side from mod_security, which I believe is an Apache function that blocks visitor actions that are potentially flagged as intrusive by our server software. Basically the server security system didn’t like something in your post. It could be a word, or more like a sequence of special characters. Basically the server thought you were trying to break in. We’ve encountered issues like this in the past that appeared with blog posts or private messages. We solved them by examining the exact content of the offending message and comparing them to the server log’s time stamps. If you or anyone encounters an error like this save your exact post content and the exact time. I can then look into it and adjust our server security settings to stop incorrectly flagging such content. Edit: looking at your screenshot it looks like something in the AdminCP. The cause is likely the same though, whatever action taken triggered mod_security.
