[imp] Deleting or moving messages with Fatal error in Socket.php

[Yves Bilgeri]

I'm testing to switch from Courier-IMAP over to Dovecot (1.2.15), but 
have a problem with Horde4/Imp5 when deleting or moving messages I get 
every time following error:

Fatal error: Call to a member function get() on a non-object in 
/var/www/horde4/pear/php/Horde/Imap/Client/Socket.php on line 2931

Horde3/Imp4 works without that problem when Dovecot is the IMAP-Server 
and also Thunderbird works well. If I switch back to Courier the 
deleting or moving works also in Horde4/Imp5 without errors.

Any hints what I do wrong?
Yves