Dear Michel.

In version 8 this was not a problem, and we have therefore downgraded to version 8. The problem is only in version 9, and by editing the source code in the file point_of_sale.py (around line 1060) - I forced the POS system to assign all unknown (anonymous) clients to be assigned to a given client. By doing this and then reconnecting the client to the POS system, all the offline tickets in the client was registered in the backend.

Before doing the hack above the tickets was only in "offline mode" in the client and not registered in the backend - it was not possible to edit the error in the backend. (It had to be corrected in the fronted (POS system) before it was registered in the backend, and in the frontend it was not possible to open a ticket that was "payed"/closed (but not yet registered in the backend) - Tried also debug mode, but the option of exporting all sales did not work either.

I will consider this a bug in the v9 POS module as I cannot find any place to set the "standard account" that has to be used to registre a sale.

For us the v9 POS solution was not stable and ready to use, and we have gone back to version 8 - which works out of the box (but lacks some features as numbering of journals that was supposed to be in v9).

Are you sure the ticket  is the reason for the error?  (not selecting a customer in the frontend is not leading to an error in my experience v8)

I would say: goto the POS backend menu

then goto the ticket where you need to correct the error (if any) and make the correction