"An unknown error occured, if you are sending mined funds, check that you have shielded them to a private address before attempting to send"
WTF???
The rule for coinbases on Verus, Zcash, and a number of other coins with zk-SNARKs, is that you must "shield" your coinbase transactions before you can use them, by sending them entirely to a z-address first, then back to a t-address. the Wallet has a shield all recent coinbases option, but you must make a private address (use legacy until Sapling activates) before you will see the option in the send screen. it is possible to send the entire amount to a z-address manually, but the easiest way to do it is by using the automatic shield coinbase option on the send screen.
There are only 2 options:
1) "Send from the private address". I created private address but there is 0 balance on it. Should I send coins to this address?
2) "Shield all recent coinbase transactions". If I understand correctly, when enabling this option all coins will be send to the private address?
For #2, it will only select mature coinbase transactions that must be shielded and leave all other transactions alone.
Where can I read information about that in details?
mature means they have gone through the required confirmations. Yea you just send them to your shielded address and then send them back.