I'm not sure if everything I'm saying here is correct, but I can only speak for myself.
A clean install of 2.3 works smoothly, but when I did an upgrade from the latest version of 2.2 to 2.3 things gone bad.
The upgrade-installation went great, but when I logged in there was no interface. No css/js and other stuff was loaded. (compressing is supported)
After comparing some system-settings I noticed that I used a custom template for the backend, I did that for personalisation of the backend with our company-logo. Logging into the MySQL database and setting the manager_theme to "default" in the system-settings did the job. Don't forget to clean your cache throught FTP and do a hard refresh on the browser.
I thought this may be helpful for others with the same issue.
[ed. note: sitsol last edited this post 12 years, 3 months ago.]
Did you try clearing the browser cache and cookies first? A leftover cookie can cause all kinds of weirdness in the Manager.
I had this on one site, though not with any custom backend template. Clearing the browser cache solved the issue.
[ed. note: absent42 last edited this post 12 years, 3 months ago.]
I would suggest to edit the post title to mention "Manager Template" instead of "Template", as it could confuse lots of users.
And 2.3 has lots and lots of manager template modifications, enhancements etc so I doubt if custom manager templates from pre 2.3 will work with 2.3.
Hope this helps.