Sadly, we cannot go to Administration → Model → Data and synchronize everything, because the menu is not opened anymore. Within the log, we cannot recognize any related error for that situation. In the old 6.8 installation the user is member of the “Administration” group.
We tried to clear some caching tables but without sucess.
This is typical of a read error on an ir.menu record usually it is the linked action that does not exist or some user access restriction.
You should look at the request from the client to detect the one that is failing and see the error message of the server.
I had this situation multiple times since I switched from Apache2/uWSGI to nginx/gunicorn. I think it happens when I install a module that creates a new menu entry and/or adds new fields to an existing model.
After restarting the server, operation is normal and the error is gone.
This does not happen when I start trytond directly (trytond -c …) and it did not happen when I used Apache2/uWSGI, so I think it is a problem with my gunicorn configuration, but I did not yet investigate.
I finally increased the log level and then inspect the tables ir_ui_menu and ir_action_act_window and found out, that one of the migration sql scripts removed the missing entry.
