I was getting the same problem after upgrading and found what might be the cause.
I have a Grandstream GXW-4108 and I have it logging to a syslog server. After the upgrade and reboot of the 3CX server, it kept giving a message about trying to reregister and failing due to a process being held, and it would retry in 10 seconds. I restarted the gateway the old fashioned way (flipped the switch), and it registered successfully.
So, for some reason, the new version is holding something open on the gateway that keeps it from releasing and connecting properly.