My client’s site began using the Square plugin as the payment gateway for EE4 back in September. I suspect that the Square token is not refreshing because for in both October and now November the Square connection has gone down. It starts working again only when they reconnected the API.
According to the Square documentation, the ACCESS_TOKEN_EXPIRED error happens when a token is not refreshed for 30 days, so that lines up with the outages every month my client is experiencing.
I can see in the code of the EE4 Square OAuth module that there is a function that is set to check if the token is 6 or more days old. I suspect that there may be a problem in either this function or the refresh token function.