5.4 to 5.6 migration, can't edit Parties

After migrating to 5.6 I find I can not open any entries I have in Parties.

I can see the list of Parties but double clicking on any of them does not change to the form.
I also can not add any new Parties although I can duplicate existing entries.

No error is displayed.

Was some new field added for Parties in 5.6 which may be empty or not populated correctly?

I guess you have custom module that modify the party form which is not compatible with series 5.6.

No, I do not have any custom modules.

I will continue to investigate.

I have not been able to find the reason for this. Enabling a high level of logging from trytond shows no errors. Is there any other debugging method I can use to see what is going on ?

Check errors on both side the server and the client.