I updated a running Leap 42.3 to 15.0. Afterwards hylafax+ fails to start:

southpole:/home/axel # journalctl  -xe
-- Subject: Unit hylafax-hfaxd.service has failed
-- Defined-By: systemd
-- Support: https://lists.freedesktop.org/mailman/listinfo/systemd-devel
-- 
-- Unit hylafax-hfaxd.service has failed.
-- 
-- The result is failed.
Apr 30 19:58:59 southpole systemd[1]: hylafax-hfaxd.service: Unit entered
failed state.
Apr 30 19:58:59 southpole systemd[1]: hylafax-hfaxd.service: Failed with result
'exit-code'.
Apr 30 19:59:00 southpole systemd[1]: hylafax-hfaxd.service: Service hold-off
time over, scheduling restart.
Apr 30 19:59:00 southpole systemd[1]: Stopped HylaFAX hfaxd (client service).
-- Subject: Unit hylafax-hfaxd.service has finished shutting down
-- Defined-By: systemd
-- Support: https://lists.freedesktop.org/mailman/listinfo/systemd-devel
-- 
-- Unit hylafax-hfaxd.service has finished shutting down.
Apr 30 19:59:00 southpole systemd[1]: Starting HylaFAX hfaxd (client
service)...
-- Subject: Unit hylafax-hfaxd.service has begun start-up
-- Defined-By: systemd
-- Support: https://lists.freedesktop.org/mailman/listinfo/systemd-devel
-- 
-- Unit hylafax-hfaxd.service has begun starting up.
Apr 30 19:59:00 southpole /usr/sbin/faxmodem[3398]: FIFO: open: No such device
or address
Apr 30 19:59:00 southpole systemd[1]: hylafax-hfaxd.service: Control process
exited, code=exited status=255
Apr 30 19:59:00 southpole systemd[1]: Failed to start HylaFAX hfaxd (client
service).
-- Subject: Unit hylafax-hfaxd.service has failed
-- Defined-By: systemd
-- Support: https://lists.freedesktop.org/mailman/listinfo/systemd-devel
-- 
-- Unit hylafax-hfaxd.service has failed.
-- 
-- The result is failed.
Apr 30 19:59:00 southpole systemd[1]: hylafax-hfaxd.service: Unit entered
failed state.
Apr 30 19:59:00 southpole systemd[1]: hylafax-hfaxd.service: Failed with result
'exit-code'.
Apr 30 19:59:00 southpole systemd[1]: hylafax-hfaxd.service: Service hold-off
time over, scheduling restart.                                           
Apr 30 19:59:00 southpole systemd[1]: Stopped HylaFAX hfaxd (client service).   
-- Subject: Unit hylafax-hfaxd.service has finished shutting down               
-- Defined-By: systemd                                                          
-- Support: https://lists.freedesktop.org/mailman/listinfo/systemd-devel        
--                                                                              
-- Unit hylafax-hfaxd.service has finished shutting down.                       
Apr 30 19:59:00 southpole systemd[1]: hylafax-hfaxd.service: Start request
repeated too quickly.                                                       
Apr 30 19:59:00 southpole systemd[1]: Failed to start HylaFAX hfaxd (client
service).                                                                  
-- Subject: Unit hylafax-hfaxd.service has failed                               
-- Defined-By: systemd                                                          
-- Support: https://lists.freedesktop.org/mailman/listinfo/systemd-devel        
--                                                                              
-- Unit hylafax-hfaxd.service has failed.
-- 
-- The result is failed.
Apr 30 19:59:00 southpole systemd[1]: hylafax-hfaxd.service: Unit entered
failed state.
Apr 30 19:59:00 southpole systemd[1]: hylafax-hfaxd.service: Failed with result
'exit-code'.

First thought:
It seems that the sendonly-process fails (Apr 30 19:59:00 southpole
/usr/sbin/faxmodem[3398]: FIFO: open: No such device or address)

southpole:/home/axel # dir /dev/ttyS0
crw-rw---- 1 root dialout 4, 64 30. Apr 19:56 /dev/ttyS0

as hylafax runs under user uucp, this user is not authorized for the interface

-> chmod 666 /dev/ttyS0 does not solve the issue

2nd thought: starting the process manually:
southpole:/home/axel # su uucp
uucp@southpole:/home/axel> /usr/sbin/hfaxd -d -i hylafax &
[1] 5653

-> works, but manual start of faxmodem fails again: 
southpole:/home/axel # /usr/sbin/faxmodem ttyS0
FIFO: open: No such device or address

Any idea what this could be?