"Internal Server Error" on the backend

[davebond]

I'm stumped - my /administrator backend is throwing the following error immediately:

Internal Server Error

The server encountered an internal error or misconfiguration and was unable to complete your request.

Please contact the server administrator, [email protected] and inform them of the time the error occurred, and anything you might have done that may have caused the error.

More information about this error may be available in the server error log.

My logs show the following error, repeatedly: 2012-11-30 17:17:54 INFO xxx.xxx.xx.xxx Joomla FAILURE: Username and password do not match or you do not have an account yet.

The frontend works fine, although users have mentioned that they've gotten the same error when trying to submit content. I use YooTheme's Zoo, but have not had an error before. This seems to be recent - within a few weeks. It's also been intermittent until earlier today.

Any thoughts?

[humvee]

1. It has not been established as far as I can see, but what were you doing before you started to get the error?
2. Is this the default Joomla login or is it a login module associated with a third party template?
3. Please specify the precise error notices that you receive, if not already included above.
4. Are there any errors listed in the log files for the server/in the joomla_root/ log files?
5. Is this a:
   1. new installation?
   2. Migration?
   3. Upgrading Joomla 2.5.x to the latest release?
6. Were you:
   1. Adding to or updating the Extensions?
   2. Changing the Template?
   3. Uninstalling/deleting extensions?
   4. Editing the database?
   5. Modifying ACL permissions for any aspect of the site or backend?
   6. Editing the Core files?
7. Transferring the files by
   • Moving the joomla_root/ directory and contents within the host?
   • Changing the host?
   • moving from a local to remote server?
8. Changing the User Authentication Methods (modifying the active Plugin)
9. Has the host updated the server?
10. Do you have a backup of the site before the issue started to occur?
11. Please run the Forum post Assistant to provide the information to help us try and resolve the issue.