If HT (high temperature) appears on your display, the problem is usually due to your dehumidifier being placed in a space where the temperature has exceeded 30 degrees (or the set value for activating high temperature).
To change these settings, you need to either disable the high temperature alarm or increase the activation value.
How you do this depends on which control panel you have.
If you have our PDC-03 panel:
To disable the function:
Hold in 
and 
for two seconds until you see 3 in the panel.
Use to get to Parameter 10.
Hold in and press until it says OF – you have then deactivated the function.
To change the temperature at which the alarm is activated:
Hold in and for two seconds until you see 3 in the panel.
Use to get to Parameter 11.
Hold in and press until you reach the temperature at which you want the alarm to be activated.
If you have our MPO panel:
Navigate to Menu -> Settings -> go down to High temp -> click on 
to access the relevant settings.
Choose between off and degrees, e.g. if you set the temperature to 37 degrees, the high temperature alarm will be activated if the dehumidifier's sensor registers a temperature above 37°.