I managed to resolve it. It was not product problem, but network issue (kind of). The sentinel server is in different network. IdM server has two interfaces. Second interface was ofcourse in same network as sentinel and they communicated just fine.
My mistake was, that posgresql at IdM was not properly binded to IP on that second interface, so it was not actually listening there (that's why the tcp connection to postgresql did not work from sentinel to idm) and second issue was, that when entering FQDN, it is resolved by IdM servers DNS, instead hosts file on sentinel, which makes no sense to me (why?! - when sentinel is the one connecting TO idm. And it has correct hosts record, why not use it?).