AAA

– Download Monitor: Version 1.4.3
– “and what are its and Email Before Download’s settings you use.” – I do not understand your message here. / I used GMAIL.
– masked option and force download option together: What’s this?
(I enabled
– X-Accel-Redirect / X-Sendfile
and
– Prevent hotlinking
within Download Manager. But no difference.)

The problem is within your plugin (Email Before Download)

<b>Warning</b>:  fseek() [<a href='function.fseek'>function.fseek</a>]: stream does not support seeking in <b>/home/flag/public_html/cmswp/wp-content/plugins/email-before-download/download.php</b> on line <b>225

Maybe I found the bug:

1. Edit email-before-download/download.php

2. Change it to the following at line with fseek:

// -- prevent adding php fseek warning within ZIP download file
	$g_CURRENT_ERROR_LEVEL = error_reporting();
	error_reporting(0);
  fseek($handle,$begin,0);
	error_reporting($g_CURRENT_ERROR_LEVEL);

3. Save.

4. A message to the plugin developer: Please update your Plugin as soon as possible.