Without seeing the actual error messages at the time of the update it is complicated to say what really happend.
You description sounds like that it hangs only at the last step where it compiles a new configuration for the configured core. This also can explain why you get different errors depending on the configured core in the system. If the system complains about problem at time of config generation like in your case, a “cmk --debug -vvU” can help to find the real reason for the problem.
The missing mgmt IP from one of your hosts you also should see in the old 1.6 if you do the same command on the shell.