Issue with mobile devices and ob_start fatal error

Hi all,

My website has recently become extremely slow with a mobile device but is working normally with a computer browser.

When I investigate on the pre-production version of the website, the website shows a blank page and when I activate error reporting it says :

"Fatal error: ob_start(): Cannot use output buffering in output buffering display handlers"
and it points the error the in /system-2-2-1/core/Exceptions.php on line 185.

The line is an ob_start() instruction in the function show_php_error($severity, $message, $filepath, $line)

Any clues ?