I want to ensure you can log in to your QuickBooks Online (QBO) account.
To fix the access denied error, I recommend first checking your login credentials. Then, let's perform some troubleshooting steps by logging in via incognito mode or a private window. This way, it confirms if the cache is the cause.
Here's how:
Press Ctrl + Shift + N (Google Chrome).
Ctrl + Shift + P (Firefox).
Control + Option + P (Safari).
If you're able to access your QBO account, go to your default browser and clear the cache to optimize its overall performance.
Otherwise, switch to a different browser like Firefox, Google Chrome, or Safari. The one you are currently on may be having a temporary issue with QuickBooks, and using a new browser for the moment will allow you to get back to work.