Issue

Cannot enable full phone encryption on LG G4 (Android 6.0). After enabling encryption in Settings > General > Security > Encrypt Phone, the smartphone restarts, but encryption remains disabled.

Resolution

This seems a bit random, but I fixed the issue by disabling Wi-Fi. After repeating the same steps to enable encryption with Wi-Fi disabled, the device still restarts, but then boots into a special mode and completes the encryption.

Some people reporting that they had to disable both, Wi-Fi and mobile data, but in my case disabling Wi-Fi was sufficient.

LG G4
Android 6.0