"ACCESS_REFUSED - Login was refused using authentication mechanism PLAIN" typically occurs when:

• An invalid password is being used
• User has not been granted read or write access

Something like this may also be found in the rabbit log.

=ERROR REPORT==== 25-Nov-2020::18:13:42 ===
Channel error on connection <0.22119.14> (10.14.115.25:39982 -> 10.97.244.15:5672, vhost: '/', user: 'john.doe'), channel 1:
operation exchange.declare caused a channel exception access_refused: access to exchange 'exchange001' in vhost '/' refused for user 'john.doe'

Invalid password

When creating a user account, the user's password will be defined. Likewise, a user's password can be reset. If the password being presented does not match the user's password, access_refused may be returned. Be aware that a Java application may read the password from a secret, such as a Docker Secret or an OpenShift Secret.

Read / Write access

Refer to RabbitMQ - Grant user permission using the Admin Console.