@jay_s_uk Thanks for that hint. I shut down the system to get a break and some food. After my breake i turned everything on and it woked after flashing the DuetWiFiServer.bin and firmware again. I was thinking about replacing that cheap china sd card while having the break but wasn´t shure because everithing else was working.
Thanks for your Answer! I will observe the system after changing to a sundisk sd and give feedback if it happends again, have a nice day.