mail module stoppped using "retrieve mail from external account"

[gx-turbo]

hello,
I did an upgrade from 3.5.5 to 3.5.9. (on ubuntu 14.04) and now mail has problems. mail did not start, everything else is ok. Whenever I tried to start mail (by web administration or by terminal), it seems to do the start, and tells me "started", no error message. But the module status itself keeps beeing "stopped". I tried some things, and found out, that it has something to do with "retrieve mail from external account". When I disable this, mail immediately starts, enabling it immediatly stops mail. Also removal of all external accounts "helps", but of course then zentyal does not what I want  .
I did not change anything other than zentyal component upgrade and system updates (from zentyal web administration).
I use roundcube as webmailer, it still works, all old mails still available. Also sending emails from roundcube still working.
Any idea?
thanks for help, Gerd

[gx-turbo]

I did some more investigation, it seems just fetchmail isn`t starting.
syslog tells me:
Jun 28 15:41:01 server cron[1314]: (*system*ebox-mail) RELOAD (/etc/cron.d/ebox-mail)
Jun 28 15:41:01 server CRON[26464]: (root) CMD ([ -x /usr/share/zentyal-mail/fetchmail-update ] && /usr/share/zentyal-mail/fetchmail-update)
Jun 28 15:41:06 server kernel: [250338.675224] init: ebox.fetchmail main process (26507) terminated with status 1
Jun 28 15:41:06 server kernel: [250338.675256] init: ebox.fetchmail main process ended, respawning
Jun 28 15:41:07 server kernel: [250338.726093] init: ebox.fetchmail main process (26510) terminated with status 1
Jun 28 15:41:07 server kernel: [250338.726126] init: ebox.fetchmail main process ended, respawning
Jun 28 15:41:07 server kernel: [250338.774001] init: ebox.fetchmail main process (26513) terminated with status 1
Jun 28 15:41:07 server kernel: [250338.774041] init: ebox.fetchmail main process ended, respawning
Jun 28 15:41:07 server kernel: [250338.812823] init: ebox.fetchmail main process (26516) terminated with status 1
Jun 28 15:41:07 server kernel: [250338.812856] init: ebox.fetchmail main process ended, respawning
Jun 28 15:41:07 server kernel: [250338.847311] init: ebox.fetchmail main process (26519) terminated with status 1
Jun 28 15:41:07 server kernel: [250338.847346] init: ebox.fetchmail main process ended, respawning
Jun 28 15:41:07 server kernel: [250338.882157] init: ebox.fetchmail main process (26522) terminated with status 1
Jun 28 15:41:07 server kernel: [250338.882198] init: ebox.fetchmail main process ended, respawning
Jun 28 15:41:07 server kernel: [250338.919825] init: ebox.fetchmail main process (26525) terminated with status 1
Jun 28 15:41:07 server kernel: [250338.919863] init: ebox.fetchmail main process ended, respawning
Jun 28 15:41:07 server kernel: [250338.956688] init: ebox.fetchmail main process (26528) terminated with status 1
Jun 28 15:41:07 server kernel: [250338.956726] init: ebox.fetchmail main process ended, respawning
Jun 28 15:41:07 server kernel: [250338.989265] init: ebox.fetchmail main process (26531) terminated with status 1
Jun 28 15:41:07 server kernel: [250338.989308] init: ebox.fetchmail main process ended, respawning
Jun 28 15:41:07 server kernel: [250339.023853] init: ebox.fetchmail main process (26534) terminated with status 1
Jun 28 15:41:07 server kernel: [250339.023892] init: ebox.fetchmail main process ended, respawning
Jun 28 15:41:07 server kernel: [250339.061448] init: ebox.fetchmail main process (26537) terminated with status 1
Jun 28 15:41:07 server kernel: [250339.061484] init: ebox.fetchmail respawning too fast, stopped



any idea what causes fetchmail to terminate ? what can I do to investigate more details ?
Thanks for any reply