the recent update of SmartCrawl brakes some other plugins.
What happens is that some ob_start() method calls ends with following error
`PHP Fatal error: ob_start(): Cannot use output buffering in output buffering display handlers in /var/www/html/wp-content/plugins/wpforms/includes/class-frontend.php on line 1089`
(at the mentioned line is the ob_start() call).
Besides WPForms (Lite as well as Pro version) it happens in my own plugins using ob_start().
So, the problems seems to be clear. SmartCrawls call ob_start() and before the output buffer ends, another (in WPForms or in my plugin) are called which cause `Cannot use output buffering in output buffering display handlers` error.
I disabled SmartCrawl right now. My tip is that the problem is in method smartcrawl_start_title_buffer() in onpage.php file, but haven't tested it properly yet.