I downloaded a few calendar and event plugins to test. I got a similar error for 2 or 3 of them:
The plugin generated 264 characters of unexpected output during activation. If you notice “headers already sent” messages, problems with syndication feeds or other issues, try deactivating or removing this plugin.
My error log shows this error:
[Mon Jun 28 06:24:29 2010] [error] [client xx.xxx.xx.xxx] File does not exist: /home/websitename/public_html/wp-content/themes/bp-daily/global-adminbar.php, referer: http://websitename.com/wp-admin/plugins.php?error=true&charsout=264&plugin=events-calendar%2Fevents-calendar.php&_error_nonce=c799c53521
The plugins I wanted to test all stated they were WP3 compatible.
Any ideas why I can't activate a calendar or event plugin?