Bogus 'Full' warning

This sounds like the tank detection switch is bad. It probably has succumbed to the high moisture environment that it is living in. The switch is inside the opening where the tank fits and serves to shut down the unit if the tank is full. If you remove the tank and look in the opening with a flashlight you should be able to identify the switch. It may or may not have a float on the end of it. Once you find it you can check it with a multimeter. If you don't have one they are inexpensive and easy to use. With the DMM on the continuity setting, and the power disconnected to the unit, check the contacts on the switch. If the switch is good it should show continuity or a very low resistance. If the switch is bad the meter will show a high or infinite resistance. Anything above a couple of ohms would be suspect. In that case get a new switch and replace it. This should solve your problem. Good luck.

Ours did the same thing. The reason I think the switch is OK is because I can reach in and push the bar (that is suspended inside the unit and makes contact with the bar/foam float in the water receptacle) and the unit runs fine.

Although our unit also ran trouble-free for a year or so before the constant "full" indicator, I have determined that the bar in the water receptacle is too short (?) to make proper contact. I know this makes no sense, esp. given that everything about the unit seems in new condition, but I added a short "extender" to the bar with paper and tape and it is working fine now.

An odd fix, but I'm not complaining anymore.

or you could just tape the bucket in place. when the compressor kicks in the unit vibrates which might be shacking the bucket loose to the point that it doesn't make contact with the switch.