Error when trying to post emojis (unicode)

One of my users tried to post something and got an error like the one in the attachment.
I tried to get the same error and it appeared when posting emojis.
After that, a thread will be created without really being written to the database.
As you can see at the screenshot, the thread publisher is the one, who posted it, but at the right, 'guest' is shown instead.
When trying to open the thread, the board tells me, it is not existing.