Access Denied when I try to flush cache or change theme This is a very odd issue. Logged in as admin. All permissions given to admin. But Access Denied page when I try to flush cache or change theme. I can flush the cache with drush. I tried switching theme to garland and back again. I tried creating a new admin user, didn't work. I tried disabling recently enabled boost module, to no avail.
I have no idea why, but I disabled clean-urls and reenabled it and now I can flush cache and switch themes without access denied.

I had installed the Boost module which also involved putting some stuff in the .htaccess and turning on clean urls (although I was using it anyway). 

This problem took me about 4 hours to solve and was completely unintuitive.