I'm commenting here hoping that my experience might help someone else who is experiencing my variation of this problem, which initially manifested in the same way OP illustrated (which brought me to this post), but was partially fixed by force reloading the broken pages. The part that wasn't fixed with the cookie clearing method involved broken menus as a persistent issue.
After I upgraded, I initially had the same broken images along with broken menus, using Firefox or Chrome for the instances which I did NOT have an Apache proxy handling auths, but the images portion of the problem didn't last long. I believe it was fixed because I have a habit of force reloading a webpage (Ctrl+F5) anytime I see broken images or menus.
With that said, even though the broken images were fixed, there continued to be certain pages/locations where the navbars and appbars are just broken, and to confuse me even more, it magically started working once, for like 2 minutes, only to break again shortly afterwards. The fastest repeatable test I found was navigating to the indexer clustering page on an instance which didn't have clustering configured yet. I tried clearing cookies, not using the fqdn, using the IP instead of the dns name, navigating to/from different pages with different contexts, modifying Cookie.py as instructed by rbal and a few other random things. Eventually I created a new host with a fresh install, thinking that running browsers side-by-side with a working instance would give me some clues to how I could fix it, but I was bummed to find that the fresh install was exhibiting the same behavior.
When thinking about my working instances, I thought that Apache handling the auth was somehow the fix, so I was going to install and configure it on my test instance, but before I did that, I realized that I had overlooked one Splunk setting which was different regardless of the proxy. The non-proxied systems weren't using https. Finally! Enabling Splunk Web SSL, and logging in with the https prefix fixed the menus... disabling it and logging in with the http prefix broke them again. Toggled back and forth between SSL and not a few times with consistent results.
Anyways, I know it was a bit long winded, but I hope it helps someone.
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