It looks like the --private-key flag binds to the viper variable IssuerKey. If I recall, we've started moving away from that in favor of the new directory approach (thus the assignment of this issue to @h2zh), but it looks like doing so might have broken backwards compatibility -- I've also seen Fabio reporting that the old param quit working for him, but can't find the message in slack at the moment.

We should do two things here:

1. Fix any backwards compatibility issues we find such that a server administrator who still uses IssuerKey gets a logged warning but can still use the param as needed.
2. Update the --private-key command to work with the new, preferred param.