Join any network & any channel, try and say anything.
mIRC sends an empty privmsg command. (debug window confirms this)
type: //msg $active some text & it works fine
Scripts are also sending text OK, only seems to affect the editbox.

Cannot confirm this.
Tried with both regular and command editbox. In all cases the privmsg contains the message that was sent.

Perhaps verify that you are not overriding editbox default behavior with scripts?

Thanks for your bug report. I have not been able to reproduce this issue. As mentioned above, is it possible that you are using a script that is causing the issue?

Ok not an mIRC bug found the culprit to be spaces.dll

Sorry

In order to prevent such problems in the future, I just filed this feature suggestion.