Thanks for the reply. I understand your suggestion and I've implemented it sometimes. However, it's an workaround and it doesn't solve the real problem.
The case is: you should point an URL to /subsite/page and it should just work on host node maped subsite. Try to map all the places in the system where this call appears is very difficult, and it can always lead to some fail.
Somebody knew about this problem and fixed it for GET requests, but it stays for POST requests.
As you can see, ad_conn -set comes a little bit later: http://fisheye.openacs.org/browse/OpenACS/openacs-4/packages/acs-tcl/tcl/request-processor-procs.tcl?r=1.105#to631
I've tried a lot of different patches without success. What I've been doing until know is try to fix the symptom, not the disease. I would like very much to see a real fix for this problem, but I couldn't find any yet.