And here is the/a solution: 
ONVIF Authentication = Off
URL: http://IP/onvifsnapshot/media_service/snapshot?channel=1&subtype=2
Subtype: 0 = Main Stream - 1 = Sub Stream 1 - 2 = Sub Stream 2
It`s working with my SD49225T-HN (Firmware 2.6.x) and FritzFon C4 (Firmware 3.92)

ONVIF Authentication = Off

URL: http://IP/onvifsnapshot/media_service/snapshot?channel=1&subtype=2
Subtype: 0 = Main Stream - 1 = Sub Stream 1 - 2 = Sub Stream 2
It`s working with my SD49225T-HN (Firmware 2.6.x) and FritzFon C4 (Firmware 3.92)