Summary
The NIF field (document) in the checkout for Portugal (UI) is allowing special characters to be entered as value. However, orders that are closed with special characters get stuck with error in the OMS.

Simulation
To simulate the problem, place an order at a checkout in Portugal, including a special character in the NIF (document) field. Then check the OMS error similar to the example above.
Workaround
As a temporary solution, we recommend that the developer responsible for the store includes a customization javascript at the checkout, validating the value entered according to the mask and the NIF rules.
Orders that have already been completed and have this error in the OMS will need to be canceled. So we suggest that the store contact the customer to explain the situation and help redo the order.