Some units have a switch called econo or miser or something like that, which turns a heater on/off for this section of the frame, check if yours has this and turn it off. It is there to prevent condensation during high humidity.

The most common cause of this is dirty condenser coils or a malfunctioning compressor fan.

Check that the fan is running properly and that it's blades are clean.

Clean the condenser coils.

I clean mine by removing the kick plate and the rear cover.
Rolling it over to an open door and blowing it out with an air compressor.
I have been told a leaf blower also works.

If this is not possible use a brush and a vacuum.
There are long brushes available at appliance parts stores for this.