I know that this thread is old, but this may help others in the same situation. My mobile is a LG G2 Mini (D618) rooted with Kingo. After I changed it from Dalvik to ART, it took a long time to reboot, while optimizing all the apps. But then, when the UI showed up, I couldn't do anything because several error messages appeared in sequence. After a while touching OK for each message, the mobile would reboot and start it all over again. In fact it rebooted even if left alone.
I re-flashed it with the stock ROM (with the no-data-loss option) but nothing changed other than it becoming unrooted again. Also tried to access it with an ADB Shell but I kept getting an authorization error. Then, after rebooting it around 30 times, it reverted back to Dalvik by itself, and everything was fine again!