I'll help you sort this out. A few times when lots of processes happening to a browser, this will make the stored cache loaded with internet files.
Thus, you can use a private or incognito browser to isolate this issue. Open a browser and press Ctrl + Shift + N (Google Chrome), Ctrl + Shift + P (Firefox) or Command + Shift + N (Mac browsers).
From there, open the Intuit Account Management portal link and turn off the 2-factor authenticator. Then switch it on again. Once done you can then login to your QuickBooks Online/Self-Employed account to test the feature.
Alternatively, you can use different browsers for Intuit Account Management portal and QuickBooks Online/Self-Employed like for example running Intuit Account Management on Google Chrome and run QuickBooks Online/ Self-Employed in Firefox to check if this a browser issue.