[imp] Login via imp successful but list mboxes leads to immediate logout

[Herbert Pophal]

Webmail Edition 5.1.3
IMP 6.2.2
PHP 5.5.24/5.6.8 (fcgid connected)
imap server: RHEL 6.6
cyrus-imapd-2.3.16-6.el6_2.5

Using these PHP versions, login (via imp) works, and horde portal shows 
up. Clicking Mail leads to immediate logout with message: IMAP error 
reported by server.

logfile says:
Nov 11 12:15:27 webhaupt-test HORDE: [imp] Login success for blafasel 
(172.26.16.4) (forwarded for [130.149.2.58]) to 
{imap://mailbox.tu-berlin.de/} [pid 24577 on line 157 of 
"/afs/tu-berlin.de/units/tubit/www/horde-test.tubit/htdocs/horde-5/imp/lib/Auth.php"]
Nov 11 12:15:28 webhaupt-test HORDE: [imp] [listMailboxes] IMAP error 
reported by server. [pid 24577 on line 726 of 
"/afs/tu-berlin.de/units/tubit/www/horde-test.tubit/htdocs/horde-5/imp/lib/Imap.php"]
Nov 11 12:15:28 webhaupt-test HORDE: [imp] [listMailboxes] IMAP error 
reported by server. [pid 24577 on line 726 of 
"/afs/tu-berlin.de/units/tubit/www/horde-test.tubit/htdocs/horde-5/imp/lib/Imap.php"]
Nov 11 12:15:29 webhaupt-test HORDE: [imp] [listMailboxes] IMAP error 
reported by server. [pid 24577 on line 726 of 
"/afs/tu-berlin.de/units/tubit/www/horde-test.tubit/htdocs/horde-5/imp/lib/Imap.php"]
Nov 11 12:15:34 webhaupt-test HORDE: [imp] [listMailboxes] IMAP error 
reported by server. [pid 24577 on line 726 of 
"/afs/tu-berlin.de/units/tubit/www/horde-test.tubit/htdocs/horde-5/imp/lib/Imap.php"]
Nov 11 12:15:34 webhaupt-test HORDE: [imp] [listMailboxes] IMAP error 
reported by server. [pid 24577 on line 726 of 
"/afs/tu-berlin.de/units/tubit/www/horde-test.tubit/htdocs/horde-5/imp/lib/Imap.php"]
Nov 11 12:15:34 webhaupt-test HORDE: [imp] IMAP error reported by 
server. [pid 24577 on line 606 of 
"/afs/tu-berlin.de/units/tubit/www/horde-test.tubit/htdocs/horde-5/imp/lib/Application.php"]
Nov 11 12:15:34 webhaupt-test HORDE: [horde] User opha4000 logged out of 
Horde (172.26.16.4) [pid 24577 on line 107 of 
"/afs/tu-berlin.de/units/tubit/www/horde-test.tubit/htdocs/horde-5/login.php"]

Preferences/Mail/Personal Information logs out as well, with same message.


PHP 5.5.18 works so far.
Any hints, except horde upgrade?


Best Regards
Herbert