In version 5.4.3 linebreaks aren't working correctly in signatures. To fix this you will need to edit 2 files.
showflat.php, line 513 and showthreaded.php line 571 currently look like this:
if ($Signature) {
$Signature = "<br><br>$Signature";
}
Change those, to this:
if ($Signature) {
$Signature = str_replace("\n","<br>",$Signature);
$Signature = "<br><br>$Signature";
}