Firefox Containers and the error "Credential Set is in invalid status:350. null"
TL;DR - Turn off Firefox Containers before reauthorizing Quicken. Or set your default browser to something else. Works with Mac and Windows.
This has been happening to me for a few weeks now, I just had the time to dig into it. The issue is that, in my case, American Express needs to be reauthenticated and Quicken takes me to their page. I select which accounts I want to use, click on Authorize, and I get a message saying "Did it, sending you back to Quicken." Once back in the app, Quicken errors out with the "Credential Set is in invalid status:350. null" error and Amex won't reauthorize.
In searching for a solution, I ran across someone using Windows who had the problem and fixed it by turning Firefox Containers off. I first tried to open both Amex and Quicken without a container, and it didn't work. I had to shut off Firefox Containers entirely for it to work right.