Webpage Login Issues — How to Diagnose I have two Macs: a 2015 iMac that lives on my desk, and a 2013 MacBook Air. I have accounts at usaa.com, and like to use Safari. While I can log into USAA's webpage easily on the Air, I'm not able to do so on the iMac...unless I switch to Chrome, which logs in correctly. 

Both Macs run macOS 10.14.6 (18G87), and both run  Safari 12.1.2 (14607.3.9); all software is up to date. Wondering if there was a corrupted cache on the iMac, I restarted in Safe Mode; in Safe Mode, and again after rebooting in regular mode, I was unable to log in using Safari on the iMac. I enabled the Develop Menu (Safari > Preferences > Advanced > "Show Develop Menu"), then chose Develop > Empty Caches to clear Safari's caches. No improvement. I trashed the Keychain entries for this webpage, and entered my login and password manually. Still no go with Safari on the iMac.

The login process is to enter my ID and password on a single page, then hit "Enter." The next page asks for my PIN, a 4-numeral number. The page after that asks for me to choose email or a text number to receive a login code. Choosing one option and hitting "Enter" modifies the screen to receive the email address or text number. Hitting "enter" then either presents a screen in which to enter the login code (Safari on the MBA, Chrome on either machine), or a screen that again asks me to choose email or a text number (Safari on the iMac). 

This is perplexing, especially since the iMac's Safari began misbehaving only two days ago, after being able to fully log in with this user ID and password and PIN for a bit more than two years and several macOS updates. No changes of any kind (updates, new extensions) have been made in the last month on either machine. EDIT: a similar but not identical misbehavior has also now appeared in a second, unrelated restaurant reservation page. Safari returns only a placeholder red dot, presumably indicating a page loading (or failing to load); in any event, the next screen (picking a date) never loads. As before, Chrome works correctly, and Safari on the MBP also works correctly and loads the page.

I see no other misbehaviors or oddities in the iMac's or Safari's operation. 

Something is awry in the iMac's Safari. How might I further diagnose the issue? Am I stuck with trying a reinstall of macOS?
@Tetsujin earns the garland.

The iMac's Safari had uBlock Origin 1.16.0 installed. When deactivated in Safari > Preferences > Extensions, Safari now loads the two problematic webpages correctly.

I'll guess that one of uBlock's data streams (it was subscribed to several, in aid of my trying to block persistent cookie-approval popups) was changed sometime last week, and began hobbling Safari. The cookie-popup blocking was very effective, but the impaired usability turned out to be a cure worse than the disease.