You got a blank output instead of the generated block so it must have failed because of "
something".
I guess that your processor now can't mine a block after 1000000 (
default) iterations because of the new difficulty since there are already 5759 blocks in your blockchain.
You can try to increase the iterations to 100000000 to extend the time before mining fail;
for
generate command (
I see, it's forked from an old version of bitcoin core):
bitcoin-cli generate "number_of_blocks" "number_of_ilterations"
for
generatetoaddress command:
bitcoin-cli generatetoaddress "number_of_blocks" "address" "number_of_ilterations"