Just thought I'd share in case anyone else runs up against this issue.
tl;dr: If you unlicense someone on Cloud, you need to transfer the ownership of their scheduled flows and re-do the embedded credentials.
One of the first things I do when I get to work is check the "Jobs" on Tableau Cloud to make sure all the automated prep flows ran overnight as expected. Usually, this is quite a boring task as the whole page is filled with green check marks and I move right along with my day. But today all I saw was a red ! with the word Failed.
Going into troubleshooting mode, my first thought was that some changes I'd been making to some flows Monday and Tuesday must have broken something. But I had successfully run those flows after making the changes so that didn't seem right. Plus, the failure was not limited to those flows--seemed like everything failed.
My first clue was seeing that on at least one flow, the error message indicated that there was a problem connecting to a published datasource and that the user didn't have access to the datasource. That's when I remembered that yesterday, I had unlicensed someone, but not deleted him from the site. The flows that failed were owned by that person.
I went to one of the flows and transferred ownership to me. It still didn't run. So I clicked "edit" to open it in browser and clicked on all the data sources, logging in to the data warehouse again. No other changes were made. When I hit "Publish," it prompted me to embed credentials, so I made sure that was clicked. Clicked "Run" again. Success! Did the same thing on one of the other failed flows: success again!
On the third flow, I wanted to find out if ownership needs to be changed or if it's just the credentials that need to be updated. I went to edit without first changing ownership and reconnected to the data. When I hit "Publish," it prompted me that I would be replacing the flow with the same name. I went ahead with this and saw that it changed ownership to me. So changing the ownership as its own step is not necessary--it will change when you republish.
You can also re-embed the credentials without opening the flow by going to the "Connections" tab on the flow's page and choosing "Edit connection..." under Actions.
Hope this helps someone!