Was this changed recently cause it seems that the issue is with the key ... As i said If you go to Admin -> Databases -> Your Database Connection and press Save are you able to save?
I received this message after I fixed another problem, the request for mysqladmin flush-hosts.
If I access one time directly from the server A (where Metabase is) to the server B (where MySQL is) via SSH shell and the CLI mysql client, with the same user used by Metabase, the problem disappear.
I don't know why and this is a bit disappointing, but it resolve the problem.
I faced this issue again last week after MySQL db container got restarted. This time it wasn't solving automatically, so I done the following to resolve it.
I have tried to access mysql docker container and using SSL certificate with trustServerCertificate=true helped me, without that I wasn't able to access the database. Thank you very much!