Seting up Roundcube as the system's mail application - problem: initial redirect

[medarion]

I'm trying to set up roundcube as my standard mail application. To achieve this, I use the "keep me logged in"-function and I tell my browser to open a specific roundcube URL:

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https://mail.oemus.net/?_task=mail&_action=compose&_to=martin%40oemus.net
This works fine, as long as I opened roundcube at least once in my browser session. Then I get the follwoing access on the webserver:

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84.179.48.73 - - [18/Sep/2012:02:00:05 +0200] "GET /?_task=mail&_action=compose&_to=martin%40oemus.net HTTP/1.1" 302 901 "-" "Mozilla/5.0 (X11; Ubuntu; Linux i686; rv:15.0) Gecko/20100101 Firefox/15.0.1"
84.179.48.73 - - [18/Sep/2012:02:00:05 +0200] "GET /?_task=mail&_id=4903027195057b985c15c6&_action=compose HTTP/1.1" 200 7154 "-" "Mozilla/5.0 (X11; Ubuntu; Linux i686; rv:15.0) Gecko/20100101 Firefox/15.0.1"
What is exactly what I want. But when I never opened roundcube within my browser instance, I get redierected to the inbox (the "plugin.summary"  is disabled and shows the inbox):

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84.179.48.73 - - [18/Sep/2012:01:58:42 +0200] "GET /?_task=mail&_action=compose&_to=martin%40oemus.net HTTP/1.1" 302 2563 "-" "Mozilla/5.0 (X11; Ubuntu; Linux i686; rv:15.0) Gecko/20100101 Firefox/15.0.1"
84.179.48.73 - - [18/Sep/2012:01:58:42 +0200] "GET /?_action=plugin.summary&_task=mail HTTP/1.1" 200 9431 "-" "Mozilla/5.0 (X11; Ubuntu; Linux i686; rv:15.0) Gecko/20100101 Firefox/15.0.1"

Does anybody know how to explain this behaviour and whether there is anyway to work around it?