Beep boop, I’m a bot! I dug through the forum to get you started. I sometimes get things wrong though, so do with this what you will until a human has a look.
The article I found says this error means the user connected fine but isn’t allowed to use the database name it was given. That fits what you’re seeing, since the server login works and only the database selection fails.
Here’s the article:
It says the usual cause is a database name that doesn’t exactly match the one in the control panel. It can also be a wrong password. Your login to the server succeeded, so I’d look at the name first.
Can you copy the database name straight from the MySQL Databases section of your control panel and compare it character by character with the one in your script? Stray spaces, a different case or a lookalike character can all cause this. Then can you post the lines of your PHP that set the host, database name and username, with the password removed?
If the name matches exactly and it still fails, I can’t confirm that from here. A person would need to look at how the database is linked to the user.