Dell Wyse 5070 sometimes after restart "no bootable device found"

Hello together,

sorry, if my english is not perfect!

I use the Dell Wyse thinclient as a proxmox server.

After installation everything works fine, till i shutdown the wyse and restart it. Then (not always) the wyse show me "no bootable device". I use the F2 option  to get into the bios. When i'm inside the bios i can directly exit the bios and the wyse will boot the proxmox system without changing something in the bios. The proxmox system is installed on a m.2 SSD.

Why does the wyse restart  90% with " no bootable device found"

I hope someone can help me with this problem.

Thank you very much!

Mosconi