Updated Armbian images v5.49 for K2. Default with kernel 4.14.52, next with 4.17.3 and DEV with 4.18.RC ... all kernels received latest possible fixes ... best to go with 4.14.y for now, while on later you might experience occasional boot troubles. Debian Stretch, Bionic is still too buggy ...

https://www.armbian.com/nanopi-k2/