I was already mounting the partitions manually, but perhaps I can better user mountopt. I will report back my findings. Thanks for your help so far.
edit: busybox.power.md5 is 707ead4a730af02adb17aa29f4c57d98
edit2: I replace busybox with busybox-original. Still in reboot loop My conclusion so far: problaby something wrong with my phone, no busybox issue.