@stephenw10 @jrey
Ok, I ordered a USB-serial cable and did a reinstall from scratch this morning.
It was easier than I thought and everything seems to be fine again.
I also saw that the router had (from factory) still UFS as file system before and that might be the reason why I had such strange file system corruptions. Hopefully ZFS is more robust now against power failures. And I will try to change the configuration so that my router is also powered by my UPS
Thanks to all,
Michael