At the bottom of the page, are you using Classic, Full, or Mobile view? Do you get this with both Quick Reply and Go Advanced? Do you get this same action when you reply and when you create a new message? Do you get this with any other forum?
And is your Message Editor Interface set to Basic, Standard, or Enhanced? You can find that by going to User CP in the top right of this page, then Edit Options on the left side panel and look near the bottom of the page.
I don't know that this is related but I do know you can change those and have different characteristics of the forum and message window.
As a related side issue, when I get a notification of reply email, there are many triple spacings. So many that I have to scroll down the page to get to the hyperlink.
Not an issue on any other website forum, and it never used to happen here.