Unexpected output during activation

[K15Lmk98]

Sorry, this is an automatic translation.

Clean installation - WP 4.9.1 - Cmb2 2.3.0 - Twentysixteen

Hello,
activating the plugin I receive the msg:
The plugin generated 6366 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.

This only happens the first time. Then the msg disappears and everything seems to work correctly.

Thanks, Claudio_

[codeminelab]

Same here.