The exception number is the unix timestamp of the time/date the fatal error occured. In the server error log (Apache log and/or php error log) a log entry should get created for each fatal error. You can now either search in the server log for the exception number (which is included in the log entry) or you could convert the timestamp to a human readable date (http://www.gaijin.at/olsutc.php) and then look for log entries made at that time.

Without more info it's quite impossible to tell you what might be the reason for the error. If the error log tells you more, you could give us more info.

Do you get this error instead of the expected index page of your site or when does it occur? Maybe it helps to repeat all steps of the instructions.

Also what's your elgg version? Do you have a backup? List of third party plugins?
 

Can we please don't guess in those "error by timestamp" threads? It gives false sense that they're even remotely similar to each other. Situation is simple. We're hiding error message from logged out and non-admins and we need to get actual error before offering a solution. People tend to follow such blind shots instead of following the right way.