Do you have skadate or oxwall?

Oxwall. But here is the weird things. I installed a copy of 1.8.0 in a new directory and everything works fine. Thinking it was a glitch someone overlooked in 1.8.4, I installed 1.8.0 in my servers root directory and I get the same ERROR.  It won't tell me what the error is or where it is, just that it won't send private messages. But it still works in my sub-directory.

Download your file "/ow_log/error.log" open it with a plain text editor and take a look at the last lines, it may contain information of message error.

Looks like a JS error, if your using cloudflare option to make js faster turn that option off. Also check your console to see the error.  If you dont know how to open the console then ask..  

Actually the problem was far more bizarre, but easy to fix. Inside my public html directory I had the core oxwall files, but I also had other sub-directories. One of which was labeled "mailbox". Now as you know the messaging system is with a mailbox directory. At the time I did not think this would conflict with anything, because the oxwall mailbox was buried within within other directories. Well! It was a problem. I had a sub-directory label mailbox for an external mail platform and apparently it was clashing with oxwalls mailbox directory which ultimated killed my messaging abilities. I renamed my mailbox in my root to email and now my messaging system works again. Don't ask me how I found this out, it was a brain fart on a whim. And it worked.

Ultimately**

Wow that is bazarre... probably something to do with friendly URL's or something, thats one reason i dont use them as servers can get confused and its such a pain to find the issue, you got lucky.. Nice job and glad you got it fixed.