Cannot edit guestbook entries

I have completely reinstalled 2.4.3 and still cannot edit the entries through the admin panel. I get the following message when I try to save an edit:

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

I can sign the guestbook and can delete entries, but cannot edit them. Please help.

I narrowed down the problem with not being able to edit my guestbook entries. If the email field has something in it I can edit the entry. If the email field is blank it will NOT let me save the edit.

Sure that's not the other way round? Would make more sense.

I'm sure. I cannot save edits to entries that have blank email fields. I also discovered that I cannot save edits to entries if the location field is blank. Neither of these fields is required in my guestbook. Can someone else try this in their database? If all fields that appear on the sign guestook page are filled in then I can save edits to entries.

You made any modifications to it?

Nothing significant. Here's the link:

http://www.marcellusny.com/guestbook/index.php

Try it with a space in there.