There are some of the reason that the form designer unable to export the data ,

1) Please ensure that the '/_files/formdesign/' folder is exist .

2) Check the folder permission , if the folder ( '/_files/formdesign/') was not grant with full permission (777) , please grant the folder with full permission.

If the form designer prompt the error message as below , you may download the patch fixes(Patch-Oneadmin42.zip) and replace the existing index files.

Warning: Cannot modify header information - headers already sent by (output started at /home/your_domain/oneadmin/index.php:110) in /home/your_domain/oneadmin/formdesign/wce.exportdone.php on line 47