I start by saying that I'm not an Apache expert, so I'll do my best for describe the strange (IMHO) thing happen.
Before all, here's the result of a telnet to the problematic site served by an Apache (IP & address are fantasy names):
telnet www.problematicsite.com 80
Connected to www.problematicsite.com.
Escape character is '^]'.
GET / HTTP/1.0
HTTP/1.1 302 Found
Set-Cookie: JSESSIONID=30A1322E0051FE076012575109C79528; Path=/; HttpOnly
Date: Mon, 03 Jun 2019 06:46:36 GMT
Set-Cookie: sto-id-%3FSaaS-A_prod%3FPMTEU02.prod.problematicsite.1701-sg=FGHIBAAK; Expires=Thu, 31-May-2029 06:46:37 GMT; Path=/
Connection closed by foreign host.
Regarding the feRedirect.do 'file' it is indeed not supposed to have any contents it only provides http headers to tell the browser to redirect to a different location using the below header:
Theoretically it's all perfect but....in some (apparently random) client PCs I can't use the http://www.problematicsite.com
url because these machines permits only the download of the zero-byte "feRedirect.do" file without a correct redirection!!
I've made some tests with the web admins and we're sure that the cause of this is NOT on the PCs clients but in the Apache server.
Sadly the web admins doesn't understand the cause...any clue?