Cannot DownloaWarning: Cannot modify header information - headers already sent b

[robertolito]

When trying to download a file from File Manager I get this error:
Warning: Cannot modify header information - headers already sent by (output started at /usr/local/cwpsrv/htdocs/admin/index.php:0) in /usr/local/cwpsrv/htdocs/resources/admin/modules/file_manager.php on line 0

Any solution for this or I have to forget about downloading files?