Would you tell me the version number and when the script fails. "faulty header" is caused if something went wrong when the script just starts to send HTML document. But Flex_DMB sends HTML document to you in various stages; when you try to login it at the first time, next when you make a messageboard, after the board is made and when a guest try to post an article, or when you login the admin page to edit the board.
Besides, you've moved. Then, have you moved all files from the old host? Or have you tried to create all messageboards anew at the new host? In either case, the old and new hosts have the same "perl path" (say #!/usr/local/bin/perl or #!/usr/bin/perl or something else)? If not, have you changed the very first line of the scripts? How about the directory or URLs at "basic set up" at the top of the scripts?