I have just migrated a site to a live server..
The site loads perfectly and all is well on the front end.
However, if I try to login to the admin, I get the silverstripe white/blue loading screen, the page stops loading (according to the browser) and the loading screen remains.
If I check the page source I can see that the html for the admin is there.
I can also add display: none; to #Loading and see the admin, but if I click any link the same occurs.
Looking at the hidden cms I can see that the right pane does not stretch to the right much, and pushes over the bottom grey bar a ways.

really not sure what to do. Works fine locally (other than every now and again I need to close FF and reopen to avoid the same 'type' of hang)

ps: I'm running the latest SS version. 2.4.2

I found a solution... but is not an answer.
If I add ?isDev=1 after admin I can login. (seems to reset it)
Now if I log out and login it appends ?isDev=1 on its own.
Any Idea what happened here?

Had the same problem today. Used your trick and it worked here too. However, i'd really love to have an explanation on the (possible) reason(s).
Any idea(s)?
I'm using 2.4.3.

The reason for it showing the admin login screen constantly is usually due to a problem loading all the required javascript modules. By doing ?isDev=1 (or a ?flush=all) you are disabling the javascript combing functionality (see assets/_combinedfiles/) so I would hazard a guess to say that the reason for your admin page not loading is due to a problem in those combined files. You should delete the folder (_combinedfiles) and try again.

I solved deleting assets/.htaccess file.

Now I can login successfully and all the js error are solved.

Thank you Willr and thank you delmi!
Deleting the _combinedfiles folder didn't solve the problem. However deleting the .htaccess file in the assets folder did. Is it ok to delete this file from a "security" point of view? Does this file only check that the fileextension of uploaded files match the "whitelist"?

Try to leave the htaccess but comment or delete the following line:

php_flag engine off

I disabled php parsing and it works. Thanks again delmi!