Internal server error on single-post page

Initially, I was getting a “no input file received” on a blank page when I clicked on a post permalink. After the usual fixes of deactivating plugins, and changing permalinks to the default setting, the single-post page loaded. But the sidebar was extremely slow to load, and I got an internal server error in the “wp meta” section. The server error looks like this:

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

Please contact the server administrator 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.

So, I removed the “wp meta” widget from my widgetized sidebar. Now I get the exact same error under the “Leave a reply” header on the single-post page, and the comment form and the sidebar do not load.

I have completely disabled the admin bar, and I have uploaded a blank .htaccess file, so I’m running out of ideas! Please help.

Thanks.