If i connect again, it works fine, so it's just the very first connection after it starts up that does this.
I think these two lines are good indications:
1-Negotiating SSH session + 2020.03.19 10:57:12 SSH Cannot load private key + 2020.03.19 10:57:12 SSH Cannot load certificate
I assume what's happening is mis is creating the key that it notices is missing, but it's only doing this after the first attempt to connect via ssh. There's probably a key file in the data dir, I would think. Maybe you can see if it's in there, try to login, see if it appears and then your second login works.
--- Mystic BBS v1.12 A46 2020/03/18 (Linux/64)
* Origin: monterey bbs (21:1/168)