This usually happens because when you go to the signature page you come back to the profile and hit submit. The signature is blank in the profile page...so you need to remove the signature from the UPDATE query in member.php
You need to modify your
member.php
In 2.0.3 find and remove:
PHP Code:
,signature='".addslashes($signature)."'
From this line:
PHP Code:
$DB_site->query("UPDATE user SET birthday='".addslashes($birthday)."',signature='".addslashes($signature)."',customtitle='".intval($customtitle)."',usertitle='".addslashes($customtext)."',email='".addslashes(htmlspecialchars($email))."',parentemail='".addslashes(htmlspecialchars($parentemail))."',coppauser='$coppauser',homepage='".addslashes(htmlspecialchars($homepage))."',icq='".addslashes(htmlspecialchars($icq))."',aim='".addslashes(htmlspecialchars($aim))."',yahoo='".addslashes(htmlspecialchars($yahoo))."',usergroupid='$bbuserinfo[usergroupid]' $awaysql WHERE userid='$bbuserinfo[userid]'");
I hope this helps.

I'm not sure what the line is in 2.2.*