Please check the apache error log. White pages normally mean php or apache errors. Maybe the plugin version does not match.
White page is maybe a misleading description... The menu is showing correctly, only the main part of the window is blank. Which Apache log file to check? I checked multiple but didn't find anything...
I cannot tell you which file there could be errors in because it depends on the system. I'd guess the best place to look is /var/log/apache2/error.log or /var/log/ispconfig/httpd/your.servername/error.log
You should try contacting the plugin author. I have no idea what's going wrong with your roundcube installation or the plugin. Maybe it doesn't match the roundcube version.
I tried to debug... the problem is the following line: $dt->setTimeZone(new DateTimeZone($this->rcmail_inst->config->get('timezone')));
My php version is: PHP 5.4.39-0+deb7u2 It looks like $this->rcmail_inst->config->get('timezone')) returns the offset, not a name as required by DateTimeZone::__construct. My problem is that I know neither roundcube nor ISPConfig enough to know how to modify it... :-(
That's definitely nothing related to ISPConfig nor directly to roundcube, but basic PHP. I would recommend replacing PHP: $this->rcmail_inst->config->get('timezone') by your timezone, e. g. Europe/Berlin or PST or whatever is at your place
I upgraded to Roundcube 0.9.5 using wheezy-backports, now the plugin works fine... Thanks for your help!