[imp] Error message in Tree.php after login Horde/IMP 4.0.1

[horde at the-universe.nl]

Hello,

I've just installed Horde 3.0.2 and IMP 4.0.1 on my test system. 
Everything works fine, however after login I receive the following error message
in the left (menu) corner: 

Warning: Invalid argument supplied for foreach() in
/usr/local/apache/htdocs/horde/imp/lib/IMAP/Tree.php on line 154

After refreshing the page, the error message disappears.

Any ideas what might be wrong? 

The mailserver is running on Qmail, Vpopmail, Courier Imap and UW/IMAP 2004.

thanks & best regards,
Jack