That's not and error but you could avoid it by using a different db user to access the database of a particular Odoo version. That will cause that one user cannot see the others users databases. The root cause is that you are using the same user for access different databases of different Odoo versions

No, that's not it.

I'm having the same issue and no matter how I try to access, I get this error.

Killing the process, starting Odoo again and immediately going (on firefox private) to /web/database/selector and choosing the database gives me this error message.

So... real answer please?

If you have only one database for this odoo instance
then add into odoo.conf :
db_name=your_db_name

it solved this issue much more easely
then adding new separate user in postgresql, grant them privileges to DB,...