Gnome XDMCP via GDM breaks when migrating from 11.3 to 11.4

We use Oracle Solaris 11.3 and 11.4 - (11.4.53.132.2 to be exact), running on T7 and S5 SPARC hardware. On 11.3, we are able to log in remotely into a gnome session using XDMCP without any issue. On 11.4 - and I tried on just about every release of 11.4 - it doesn't work. Just a black screen hanging - sometimes indefinitely, sometimes till a timeout. 

On 11.4 it's GDM 41.3 

On 11.3 it's GDM 2.30.7

Logs contain a variety of things that may or may not be the cause here. Very suspiciously, on the "good" 11.3 machine /var/log/gdm has a few files like 192.168.1.221:2-greeter.log and 192.168.1.221:2-slave.log while on the "bad" 11.4 we see only :5.log . Still, new logfile is created and port 6000 can flow between X-station and servers