%Error opening nvram:/startup-config (Unknown error 0)

One of these good morning messages you do not like on a Cisco Router, especially if it is followed by a more cryptic message like NV: Invalid Pointer values ...

The solution to fix that problem was a corrupted NVRAM which can be fixed by executing erase nvram:

Don’t forget to write your running config to your startup - so you have not worry to much about reboots.