managesieve server not found

[Matthias Busch]

Kolab 3.4 (upgraded from 3.3.) on Debian 7 x64


Hi,
I noticed that there is a problem with the filter. apparently 
managesieve server is not running

steps to reproduce for me
- log in webmail
- go to settings --> filter

error pops up : "an error accorued. can not connect to managesieve 
server" (paraphrased, my error is in german)

this is happening on two different machines.

any idea where to start looking?
any log files which might help?
or better yet, know how to fix?

any and all help will be appreciated!