Hi JP,
I believe this is a former bug. Please update your files with the latest available and let me know if the problem continues. The "admin-30" is the error number I use for debugging. It means the error occurred in admin.php and allows me to find the exact line it occurs.
The bug was basically a problem with failing to global and specify the $link var for mysql_query() in a common function that reads custom fields for a list:
function getcfields($list,$xtra=''){
global $ltable, $link;
if($xtra=='users') $terse = 1;
$cmd = "select field1,field2,field3,field4,field5,field6,field7,field8,field9,field10 from $ltable where listnum = '$list' limit 1";
$lrow = mysql_query($cmd,$link) or die('admin-30-'.mysql_error());
Regards