I've read some of the posts above and realised that the meddling with the mission briefing text is done by SH3 commander, so it's just as likely the problem lies there. I have looked at the "Menu variable_en.txt" file that comes with SH3 commander and I can't see anything obviously wrong with it.
Whilst the text is waaay longer than the vanilla game expects I assume that it all worked at some point and something has changed to break it. Is the bug linked to a particular edition of SH3 Commander? Or maybe a SH3 mod?