Thank you Hirama666 for the screenshot.
This is an AJAX HTTP error with status code 423 Locked:
423 Locked (WebDAV; RFC 4918)
The resource that is being accessed is locked.
I've never had this error myself. It may be related to WebDAV.
Is the error happening when you are writing or when you press the "Send" button?
You may have a more specific error message in your Apache
error.log file. Please reproduce the error and post the message.