Upon activation getting an error on Multisite install

WP 5.4, Multi-site. After network activation of the latest versino, I’m getting an error “The plugin generated 181 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.” please advise. Previously this plugin also conflicted with Advanced Custom Fields plugin. I’ll create another item about that.