Permanent "booooo" when mining

I'm trying to clone altcoin.
Now I proceed to the most important step - premining coins.
So I connected two nodes by LAN, created conf files, like this

Code:

rpcuser=host1
rpcpassword=hostpwd1
rpcallowip=*
daemon=1
server=1
listen=1
port=15814
rpcport=15815
addnode=192.168.1.100
addnode=192.168.1.101

After both clients connected I start mining with cpuminer like this

Code:

./minerd -o http://127.0.0.1:15815/ --userpass=host1:hostpwd1 --coinbase-addr=*******************************

Miner starts, works. But time to time it throws messages "booooo", and never "yay!!!". So all blocks rejected by server.
Example of output

Code:

[2018-03-20 16:34:18] thread 0: 16404 hashes, 3.27 khash/s
[2018-03-20 16:34:23] thread 3: 15396 hashes, 3.06 khash/s
[2018-03-20 16:34:23] thread 2: 16512 hashes, 3.31 khash/s
[2018-03-20 16:34:23] thread 1: 16332 hashes, 3.27 khash/s
[2018-03-20 16:34:23] thread 0: 16368 hashes, 3.28 khash/s
[2018-03-20 16:34:28] thread 3: 15300 hashes, 3.18 khash/s
[2018-03-20 16:34:28] thread 2: 16548 hashes, 3.19 khash/s
[2018-03-20 16:34:28] thread 1: 16356 hashes, 3.27 khash/s
[2018-03-20 16:34:28] thread 0: 16392 hashes, 3.27 khash/s
[2018-03-20 16:34:33] thread 3: 15924 hashes, 3.23 khash/s
[2018-03-20 16:34:33] thread 2: 15972 hashes, 3.15 khash/s
[2018-03-20 16:34:33] thread 1: 16344 hashes, 3.27 khash/s
[2018-03-20 16:34:33] thread 0: 16356 hashes, 3.27 khash/s
[2018-03-20 16:34:36] thread 0: 8088 hashes, 3.27 khash/s
[2018-03-20 16:34:36] accepted: 0/6 (0.00%), 12.92 khash/s (booooo)

What can be wrong? Is it ok, just need more time?
All the solutions I've found on the internet says that you using wrong --algo. But that's now my case.
I don't know any more probable reasons to cause that.