Looking at your picture, it is showing a black editbox... but the editbox line is white. This should not be possible in the latest beta. The editbox line should be a colour close to the editbox color. Are you using the Monochrome theme in the color dialog? Can you please type the following in your status window and report the result?

//echo -a $os $version $iif($beta,04 $+ $beta $+ ) $md5($mircexe,2) $file($mircexe).sig $alias(0) $script(0) $dll(0) $com(0) $hmac($mircexe,2)

I have still not been able to reproduce this issue.

Can you please describe a step by step method that reproduces the issue for you?

For example, here is what I tried:

1) Run mIRC
2) Disable Treebar in View menu
3) Move switchbar to the left of the mIRC window
4) Maximize the status window
5) Connnect to a server
6) Join a channel
7) Type Alt+Q to show the second editbox

When I do the above, the editbox lines are colored correctly.