Definitely not the same as the other site. This one has very few auto actions and all of them are triggered off the user saving their profile, which obviously comes long after logging in. I suspect a recent update of some extension is causing the issue. However, this site is pretty simple. There is nothing "fancy" going on with it. It doesn't even have that many extensions installed.
The most recent updates within the last month are (in order newest to oldest):
- CB (incl. all professional add-ons)
- JCE Pro
- Akeeba Backup
- Admin Tools
- Regular Labs Tooltips
- Regular Labs Components Anywhere
- googl Shortener (I don't know what installed this)
- JEvents
- Joomla (3.9.24)
- Regular Labs Better Preview
All of these have been disabled individually (except for any CB extension) and in each case doing so made no difference. Both Akeeba and Admin Tools have been fully uninstalled and, again, doing so made no difference. I know logins were working when J! 3.9.24 was released and updated on the site.
Something else notable: I put the site in offline mode and I can log into the front just fine as super-admin.
So although CB may not be a "time bomb", it certainly appears something is. And because the site uses the CB login module and I have only NOT disabled CB extensions, it seemed like the right place to start.