Client domain's webmail gives blank page

[fossxplorer]

Hi,
today i discovered that webmail.clientdomain.com gives a blank page. Checked the Apache logs and i can see error code 500. I'm using PHP55u with PHP-FPM_event. Created a simple PHP file with phpinfo() inside client domain's docroot and it works fine. Kloxo-MR web panel also works fine. I've run sh /scripts/fixweb/fix-all/restart-all without any luck. I got it working with PHP54 and mod_ruid2, but setting back to PHP55u and PHP-FPM_event, it works for a while and then gives blank page again.

Any clues? 

[MRatWork]

Look like it's related to php rather than apache. Enable display error and you will see error.

[fossxplorer]

Thanks for your prompt reply 
I've enabled display_errors (on) and error_reporting = E_ALL in /etc/php.ini, restarted php-fpm, but  still blank page and no output/
Just read that Roundcube enforces  E_ALL &~ (E_NOTICE | E_STRICT), not sure about it's E_ALL inclusive though.

[fossxplorer]

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.

[MRatWork]

Maybe they are not compatible php55u. Try using php54 or php53u.

[fossxplorer]

I got it working finally
Thanks!