If you try to load the admin page, but the page doesn't load, but the shop itself does work, it could be that your host is playing with the servers.
I was like OMG what did I do wrong when this happened to me, but after talking with my host I knew enough. They were updating to PHP5.2.4 and this went wrong cuz of a bug in the PHP_Self value (they said..) where it duplicates this value.
To keep the shop running, the solution it to switch back to php4. (via HELM control at my host, don't know about yours though) and wait a couple of days till the bug gets fixed.
I hope this information will help you when stressed out seeing a blank admin page!
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