and what xoops,php and sql version do you use?
its no bug from the form.php, that has nothing to do with it - i havent seen this problem before so i cant say anything.
because this problem can only happen when something went wrong the insert or update command (sql).
do you have sql problems generally? do you have problems with other modules regardings sql things? do you also get the error message when you update a category (change something)?
edit: try to do the following thing, open this file-> pd-downlaods folder/admin/category.php
go to line 271
and change this Quote:
('', $pid, '$title', '$imgurl', '$description', '$nohtml', '$nosmiley',
to this Quote:
($cid, $pid, '$title', '$imgurl', '$description', '$nohtml', '$nosmiley',
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