To my usual readers, you can ignore this. This is simply my way of making sure that the next time I have to google an issue, or someone else days, there’s a better chance of finding a solution. Note, there are other places posting the same solution.
But simply put, over the weekend client’s IT group rebooted a server and an FTP process started to fail. It took a lot of digging to solve it.
It appears that two things happened:
The IUSR to the FTP root directory got munged (it’s not 100% clear and this may not be a necessary step. But, things did NOT work for me until I did this, but then they continued to work when I removed the IUSR to see if I could recreate the problem.)
The part that was 100% necessary was this: https://manage.accuwebhosting.com/knowledgebase/941/FTP-Error-530-User-cannot-log-in-home-directory-inaccessible.html go to Step 8 and add all users. Why this was removed, I don’t know. But adding this after re-adding the IUSR step seems to have solved the issue.
That said, before anyone asks, “why in the world are you using FTP in the 21st century?” I won’t disagree other than to say this is purely an internal process that simply moves some non-PII data to a 2nd server. Not a huge justification, but there it is.