Ok, finally, I got it...
My shitty windows system kept me from seeing reality.
- my network configuration actually cached DNS replies, I then fiddled with the settings until the result on the screen matched with the DNs replies in NSlookup and dig
- Firefox cache is not refreshed if you reload/refresh the page. This is almost criminal in my eyes. Please hang the guy responsible for this. As a result the size of my cache is now 0.
Ok, finally the correct procedure was this one:
1- create a standard site on ISPC3, domain.com, with the standard DNS zone from the template
2- inside that site, create (for example by FTP) a /subdomain/ subfolder inside /www
3- create a new subdomain on ISPC, with host 'subdomain' and domain 'domain.com' and with redirect type "L" to directory /subdomain/
4- create an A-type record associating subdomain to the IP in the DNS Zone of domain, in the same manner as mail or www
Thank you Croydon for all your help and your patience.
I didn't expect that Windows XP TCP/IP networking would let me down, and even less did I expect such behavior from FF3.