Electrical cable getting warm
If it is only getting hot at the plug end and not along the rest of the cord then the plug itself may not be getting a good connection at the outlet.

This causes a high resistance connection which generates heat when current is drawn by the heater.

Make sure the prongs on the plug are clean and make a good connection with the outlet. Try the plug in a brand new receptacle if you have one or a new power strip. If the plug is worn and feels loose in the outlet then you could replace the cord cap with a new one.

Also, this could be a problem with older receptacles that are worn out. In older homes the receptacles get sloppy and cords can be inserted and removed too easily. This also creates a high resistance connection. With a small load like a lamp, it is not as much of a problem, but with a heater that draws high current it can become more of an issue. The solution here is to replace the receptacles.