Another user was facing an error this week when the STB was downloading OTA update (that was a SD box though). The issue was resolved after a Factory Reset of the STB. It might work for you as well. Do a factory reset and then switch OFF the STB power supply and switch it ON again.
Do you have HBO HD subscription? If yes, could you please check if the subtitles are being displayed now? If they are still NOT appearing, please try using the Subtitle button in the remote control to check if it enables/disables subtitles for this channel.
Do you have HBO HD subscription? If yes, could you please check if the subtitles are being displayed now? If they are still NOT appearing, please try using the Subtitle button in the remote control to check if it enables/disables subtitles for this channel.