You could have found more detail on the actual error by checking your Apache error log. I have realised the default error message does not help at all. If you do have access to an error log this will usually give you an idea of what may be wrong.

I have the same error when I upgraded from 2.3.1 to 2.3.3
I solved this problem by appending ?flush=1 after /admin/

i.e. http://server.com/admin/?flush=1

I got at the same stage the same error message and saw a very quick blink of a larger error message site...
after going to www.mywonderfullsite.com/?debug=1 message site came again and showed me that I had some double files
like:

Warning: Warning: there are two 'EditableButton' files both containing the same class: '/usr/www
/users/packag/userforms/trunk/code/editor/EditableButton.php' and '/usr/www/users/packag
/userforms/code/editor/EditableButton.php'. This might mean that the wrong code is being used.
in /usr/www/users/packag/sapphire/core/ManifestBuilder.php on line 276

I skipped the files and now it works fine.... but still I need a place to put in the files for the contact form.
Hope this helps!

Had the same issue after deleting a page and attempting to db/build/?flush=1. By looking at the Apache error log ("apache/logs/error.log"), i found that the MySQL temp file was locked. Stop Apache and look in the "MySQL/data" folder. Delete files listed in the Apache error log (probably look like this #sql_????_?.MYD) and restart Apache.