Your new server has a more recent PHP version which utilized a function that did not go well with older s9y versions. You will now need to update your serendipity versions to at least 1.1.2.
You could also manually patch the include/compat.inc.php file to comment out the region where input_name_to_filter is used, but you need some knowledge to do that.
Regards,
Garvin
# Garvin Hicking (s9y Developer)
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