Swift-mailer plugin can't be activated

I am using SendGrid Wordpress plugin together with Swift Mailer plugin to send SMTP emails. After moving the site from OpenShift to DigitalOcean, Swift-mailer plugin can't be activated.

Error shown: The plugin generated 185 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.

It is a fresh LAMP server.

Any suggestions for solving this problem?