EXE download error from our web site

Some EXEs from our website are not downloading completely.

(ex: ftp://www.winmansoftware.com/files/WinmanCAERP_Update.EXE)

Error file link is: http://incometaxsoft.com/err.gif

Error is coming in less than 1% of our customers’ Pcs. For remaining customers same files get downloaded properly.

We tried various options as mentioned below. (but in all the cases problem persisted)

1. Tried with different browsers - chrome, Firefox.
2. Cleared DNS configurations and IP reset
3. Uninstalled Client pc Antivirus
4. Disabled our web server antivirus.
5. Error comes in our own download utility (using winsock) also.
6. We restarted our web server.

When we compressed our exe (as .zip) file got downloaded & got extracted. Our

We are using “Windows Server 2008 R2” in our web server. We have 2 web servers and both are of same configuration.

Server configuration:

OS: Windows Server 2008 R2

Antivirus: Symantec  Endpoint Protection 2012

Client pc configuration:

Operating System Version - Windows XP Service Pack 3

Antivirus: Avast in one pc & Kaspersky Internet security 2013 in other pcs

Internet connection: Wired Broad Band with automatic IP configuration

Internet Explorer Version - Internet Explorer 6.0.2900

In summary

1. same file can be downloaded from Dropbox
2. If we compress it gets downloaded from our website itself,
3. In lots of other computers can download exe itself without error (more than 2500 PCs have downloaded it & error comes in around 5 PCs)

If it was web server issue then other customers could not have downloaded. If it was customer PC issue they could not have downloaded from Dropbox.