This is a rare problem, but I deleted the error log because I had fixed a problem that had filled my log up (LDAP related), and so I deleted the old log file. When I tried to go to the admin page without an error log file, it returned a 500 error, and I couldn't access any admin functions. Maybe a check to make sure the file exists would be good before trying to load the contents of the file.
Fixed in 2.6.