Some months ago I got one error with Oracle Data Guard and now I had time to review it again and write this article. Just to be clear since the beginning, the discussion here is not about the error itself, but about the circumstances that generated it.
The environment described here follows, at least, the most common best practices for DG by Oracle. Have 1 dedicated server for each one: Primary Database, Physical Standby Database, and Observer. The primary and standby reside in different data centers in different cities, dedicated network for interconnecting between sites, protection mode was Maximum Availability, and runs with Fast-Start Failover enabled (with 30 seconds for threshold). The version here is 12.2 but will be the same for 19c. So, nothing so bad in the environment, basic DG configuration trying to follow the best practices.