bitcond v0.12.1. "UpdateTip: 6 of last 100 blocks have unexpected version" I replaced my bitcoind from v0.11.2 to v0.12.1. Then I found "UpdateTip: 6 of last 100 blocks have unexpected version" in debug.log. This error occurs at new block generation. Can I ignore this? If not, how can I fix it?

I downloaded the new bitcond from 

https://github.com/btcdrak/bitcoin/releases
https://github.com/btcdrak/bitcoin/releases/download/v0.12.1-addrindex/bitcoin-0.12.1-addrindex-linux64.tar.gz
Yes. You can ignore this.

This means that some miners have mined (still compatible) blocks with a version number unknown to Bitcoin Core.

Generally this indicates that some miners have mined blocks that signal non Bitcoin Core compatible soft- or hard-forks. Very likely a 2MB hardfork after BIP109 implemented by Bitcoin Classic.