[imp] Blank page instead of INBOX listing

[Otto Stolz]

Hello,

I am testing Horde 3.1.3, Imp 4.1.3 + Turba 2.1.3
under PHP 5.2.0 and Linux (Suse 10.2).

I have configured two IMAP servers.

Two users of one of those servers report that they see a blank
page after log-in. One of them using Mac/Safari from an IP address
within our LAN, the other one Windows/Firefox from a T-Online
dial-in connection.

Other users of that same IMAP server (including my test account)
can view their INBOXes without any problem, and no similar problem is known
from the other IMAP server.

The Horde log has:
> Mar 22 20:16:17 Horde3test [notice] [imp]
>     Login success for xxxx [84.159.78.105] to {mail.inf.uni-konstanz.de:993}
>     [on line 152 of "/var/srv/www/htdocs/webmailtest/horde/imp/redirect.php"]

... and that is it.

Apache`s  SSL request log has:
> [22/Mar/2007:20:15:45 +0100] 84.159.78.105 TLSv1 DHE-RSA-AES256-SHA
>    "GET /horde/js/horde.js HTTP/1.1" 7070
>    "https://webmailtest.rz.uni-konstanz.de/horde/imp/"
>    "Mozilla/5.0 (Windows; U; Windows NT 5.1; de; rv:1.8.1.3) Gecko/20070309 Firefox/2.0.0.3"
...
> [22/Mar/2007:20:16:18 +0100] 84.159.78.105 TLSv1 DHE-RSA-AES256-SHA
>    "GET /horde/imp/mailbox.php?mailbox=INBOX HTTP/1.1" -
>    "https://webmailtest.rz.uni-konstanz.de/horde/imp/"
>    "Mozilla/5.0 (Windows; U; Windows NT 5.1; de; rv:1.8.1.3) Gecko/20070309 Firefox/2.0.0.3"

where the "-" (instead of a number) means that no info was
sent beyond the HTTP headers.

The PHP error log has nothing.

I have asked the pertinent IMAP server's administrators for their
pertinent log entries, but have no answer yet.

What else can I do to locate the problem?

Thanks for any hint,
    Otto Stolz