1. We have added logs to IpnController: Mage::log("PayPal IPN ".time().".", null, "paypal_ipn_access.log"); but no logs are written unless we open URL with browser or send POST request ourselves.

2. If I build POST request with a form using data of the IPN history and send it. It works:

"POST /paypal/ipn/ HTTP/1.0" 302 916 "-" "PayPal IPN ( https://www.paypal.com/ipn )"
"POST /paypal/ipn/ HTTP/1.1" 200 298 "http://otherdomain/test.html" "Mozilla/5..."

 The second one works

3. We have made some hacks to magento: 4 decimal digits and because of this we had to make some changes to avoid paypal suspect fraud.

I don't understand why paypal IPN call dont get to the controller. It is redirected first.