Constraint Error

Hello,
I am using your great plugin in free version and it’s really good, I will upgrade soon for pro version to get more options !
But I have an issue when I am typing a client name in the search customer field.
Somes times, a pop up error window appear with this message : ConstraintError “A mutation operation in the transaction failed because a constraint was not satisfied”
See link below to get a screen shot : https://www.kidacom-online.com/intrepides-jura2019/wp-content/uploads/2020/01/error-screenshot-pos.jpg
Sometimes (like today) I get another different message error about “email key”, see screenshot below :
https://www.kidacom-online.com/intrepides-jura2019/wp-content/uploads/2020/01/error-2-screen-shot.png

I dont understand these errors and how to avoid these messages, could you help me please ?

Thanks in advance.

Claire