Odd behavior when signing in to QuickBooks Online is sometimes caused by a browser-related issue. The stored cache can affect the performance of the program.
To rectify this, I'd recommend accessing your QuickBooks account via a private window. Here are the keyboard shortcuts:
Safari: Command + Shift + N
Google Chrome: CTRL + Shift + N
Mozilla Firefox or Microsoft Edge: CTRL + Shift + P
If it works, let's clear the browser's cache. Doing so removes its stored cache to ensure the program performs efficiently.