Here's my particular experience in case it helps others, but understand not everyone is in the same boat.

IF your QBO works fine in Incognito Mode, but does NOT work in a normal browsing session, here is something you should check in your troubleshooting efforts.

The primary culprit in my case was the Malwarebytes Browser Guard extension in Chrome.

I noticed on another site I was having similar issues with pages/dropdowns being blocked. I started going through and disabling extensions thinking there had to be something going on with an ad or page blocker. I had AdBlock disabled already.

Disabling Malwarebytes Browser Guard corrected the "dropdowns not working in QBO", in my case, but if you can't get it working in Incognito either, then that's probably a different issue.

I typed chrome://extensions into my address bar and started turning off extensions that had "blocking" properties such as AdBlock and MalwareBytes. Other virus scanners like AVG may also have browser guards that may need to be shut off.

It's not a great solution, since it's important to have anti-malware on a browser for safer browsing, but it was interfering with multiple sites, including QBO.

@Mark_R - I suggest in Intuit's standard troubleshooting response playbook to also consider browser extensions that could be interfering. "Clear cache" is a pretty broad first level "fix", similar to Microsoft Windows' constant advice to use SFC /scannow as a first line fix, but browser extensions were a big one, at least for me.