If your browser only displays an error message, Annotate (or one of the components it relies on) may have encountered a problem. If possible, Annotate will display an understandable error message so that you can track down the problem and fix it.

If you are only getting an Internal Server Error, chances are that you can find further advice in the log file of the web server (e.g. /var/log/httpd/error_log).

The online version of Annotate might provide helpful comments, especially when someone encountered the same problem as you before.

If nothing helps, please feel free to contact the author of Annotate.