1 - You try to edit an existing invoice and you are unable to double click on it, or open it by clicking on the edit button. 2 - Or you try to create a new invoice and nothing happens.
The most likely cause is that the terminal was once set up with a Credit / Debit card pin pad attached.
Verify that the client is not sharing terminal numbers on different PCs.
Then check the setup wizard and if one is set up in the payment processor setting, set it to None.
Verify that it is now fixed by opening another invoice, or try to edit one.