How to Remove Old Events from the Managing Events Tab
The error is likely misleading. You'll find more details in your CiviCRM error log (and/or perhaps your CMS error log). The CiviCRM Log Viewer extension is the simplest way to see those.

It's likely that these old events have some configuration that is blowing up when you resave them with the end date.

Since you just want to hide them, your simplest course of action is to set an end date directly using sql, e.g. with something like:

UPDATE civicrm_event SET end_date = start_date where isnull(end_date) and start_date &lt; '2013-01-01'