Roundcube doesn't work in cross partition environment

[EthraZa]

Hi!
 In the scenario:
 

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Home dir: /home/user/public_html
Mount command: mount -bind /mnt/sdb/user_html /home/user/public_html The Round Cube 0.2.1 is on user_html.

 I have other php systems running in the same directory, only Roundcube doesn't work. It prompts a plain white screen. The problem seems to occur on line $RCMAIL = rcmail::get_instance(); in index.php, because any echo before that prints out on the white page.
 I tried without the mount and it worked, so it is clear that have some problem with mount or cross-partition thing.

 Just for the records, I'm using that mount, because this users is on another disk that has drbd running on, and proftpd chroot jail, doesn't like symlynks. And I'm running under Apache MPM Prefork 2.2.11 with mod_php 5.2.9 on a Mandriva Linux 2009.1 x86_64.

 Thanks 4 n help!