I've just found another bug.
In lang_main.php, around line 1306:
$lang['Tell_Friend_Body'] = "Hi,nI just read the topic >>{TOPIC}<< at {SITENAME} and thought you might be interested. Here is the link: {LINK}nnGo and read it and if you want to reply you can register for your own account if you have not done so already.";
When a user uses the "Tell a friend" form, everything after the "<<" sign is lost.
You could change that line this way:
$lang['Tell_Friend_Body'] = "Hi,nI just read the topic «{TOPIC}» at {SITENAME} and thought you might be interested. Here is the link: {LINK}nnGo and read it and if you want to reply you can register for your own account if you have not done so already.";
And that solves the problem...