Guys, do you know, whether it's necessary to have different private keys for each miner, if I am mining on 1 machine on multiple threads?
Do not start multiple instances with the same key, either on the same machine or different machines.
You can have as many threads as you want on a single instance with the same key.
The signature nonce is constructed by the signing key and the thread number. Different instances will have the same thread numbers (1, 2, 3, etc.)
Your best bet is for each steemd you run, you need to have a different key. If in doubt make a different key.
I haven't looked to see if the devs fixed this yet.