I've with will introduced an error in a file inside client domains docroot and i can see the error displayed on the page, so error_reporting = E_ALL works fine.
Tested with Afterlogic, same issue so it's not limited to Roundcube. I'll investigate a bit more.
My findings so far using PHP55u and PHP-FPM:
Afterlogic error: blank page, but error in the log:
FastCGI: server "/home/kloxo/httpd/webmail/afterlogic/webmail.clientdomain.com.1fake" stderr: PHP message: PHP Fatal error: Call to undefined function json_encode() in /home/kloxo/httpd/webmail/afterlogic/libraries/afterlogic/common/managers/integrator/manager.php on line 957.
Roundcube: blank page, no error in the log
SquirrelMail: works fine!
N-th update/edit: might look like php55u-pecl-jsonc was the missing link, but i will let it go for some time and see.