Connection to AWS database gives back "You may not specify an IPv4 CIDR for an existing referenced group id rule."

Question

I try to connect to a AWS database instance from a client by following this tutorial:

https://medium.com/simplebackups/allow-a-remote-ip-to-connect-to-your-amazon-rds-mysql-instance-2d5074d601eb

I looked for my own IP and pasted in "Source" as suggested by the website. However, there is the error below. Could some of you help me to understand it?

You may not specify an IPv4 CIDR for an existing referenced group id rule.

Best regards, Bruno P.