Why does the FTP file transfer error occur due to the vk.com widget?

Question

HTML

This is not the first time I have encountered this problem, and not necessarily because of the vk.com widget.
The bottom line is this:
There is a file on the server, insert the following line into it:

<script type="text/javascript" src="//vk.com/js/api/openapi.js"></script>

If you upload this corrected file, then it is uploaded, but only part of it (for example: there are 87 lines in the file, but 55 will be downloaded)
FileZilla displays:
***
Status: Connection established
Status: Starting to upload /tmp/fz3temp-1/script.php
Command: CWD /www/path
Response: 250 CWD completed successfully
Command: TYPE A
Response: 200 Type set to A
Command: PASV
Response: 227 Entering Passive Mode (5,9,154,17,132,201).
Command: STOR script.php
Response: 150 Opening ASCII data mode for script.php
Response: 450 Transmission cancelled. Lost connection to file server
Error: File transfer failed
Status: Starting to upload /tmp/fz3temp-1/script.php
Status: Getting directory listing "/www/path"...
Status: Skip loading /tmp/fz3temp-1/script.php
Status: File uploaded successfully, uploaded 2.3 KB in 10 seconds ----- here it is uploaded part of the file
***
xfirelog gives:
***
Tue Apr 05 06:54:10 2016 0 test.ru 1419 /var/www/script.php a _ ir partspro ftp 0 * i
Tue Apr 05 06:54:15 2016 0 test.ru 1419 /var/www/script.php a _ ir partspro ftp 0 * i
***
proftpd.log says
***
Apr 05 07:00:20 Debian-60-squeeze-64-ISPLite proftpd[ 16688] Debian-60-squeeze-64-ISPLite (test.ru[::ffff:94.94.94.94]): USER sflp: Login successful.
Apr 05 07:00:21 Debian-60-squeeze-64-ISPLite proftpd[16688] Debian-60-squeeze-64-ISPLite (test.ru[::ffff:94.94.94.94]): FTP session closed.
Apr 05 07:05:51 Debian-60-squeeze-64-ISPLite proftpd[16497] Debian-60-squeeze-64-ISPLite (test.ru[::ffff:94.94.94.94]): FTP no transfer timeout, disconnected
Apr 05 07:05:51 Debian-60-squeeze-64-ISPLite proftpd[16497] Debian-60-squeeze-64-ISPLite (test.ru[::ffff:94.94.94.94]): FTP session closed.
***
I don't know where else to dig in what logs to find an error.

Answer 1

The only thing that can be (related specifically to the html script tag) is that there is an antivirus on the server that thinks that "//vk.com/js/api/openapi.js" is a virus.
But since you said that this happens not only with the VK widget - the problem is clearly not in the content of the line

<script type="text/javascript" src="//vk.com/js/api/openapi.js"></script>