1) Update to the latest KKBOX iOS from App Store.

2) Switch from cellular data to Wi-Fi or try connecting to another Wi-Fi hotspot.
※If KKBOX only works normally when connected to Wi-Fi, tap the device's [Settings] → [Cellular] and turn cellular data on for KKBOX.

3) Quit KKBOX and restart your iPhone.

4) Switch to Offline Mode and try playing your Offline Songs.
→You CAN play Offline Songs.
Ensure you have a stable network connection (refer to Step 2).

→You CANNOT play Offline Songs.
The audio file may be corrupted. Try deleting 2 to 3 tracks and download them again.