Blown fuse from a dehumidifier that has been working for years
When you say you have not plugged anything new in, I take it that you have not bought any new appliances or electronic gadgets. In addition, you stated that the dehumidifier functions just fine when plugged into another outlet. If that is indeed the case then read on. 

I have seen something similar occur twice in both residential and industrial settings. What may be happening is a bad connection in the wiring is causing a voltage drop. The bad connection might only affect that one outlet, the whole circuit or part of a circuit.

A small load like a lamp or phone charger will function properly and a voltmeter will show the outlet/circuit as good. But a larger load like a vacuum cleaner or dehumidifier will draw enough current to drop the voltage down enough to where the motor can't get up to speed causing it to pull lots of current. The breaker can't handle the higher current so it trips.

Try this test:


Unplug the dehumidifier from the suspect outlet.
Plug a lamp with an old school incandescent bulb into a different, known working outlet. Note the brightness of the lamp.
Now move and plug the lamp into the suspect outlet. Turn the lamp on and let it sit for a few minutes and note the brightness. The lamp should glow brightly and the circuit breaker should not trip. NOTE: This will NOT reliably work with a compact fluorescent (CFL) or LED bulb.


If the breaker trips or you immediatly notice that the lamp is glowing dimmer in the suspect outlet than the good outlet, you have a wiring problem. If the lamp works fine then proceed below...


Plug the dehumidifier into the suspect outlet ALONG WITH the lamp and turn it on. Make sure the lamp is on before you switch on the dehumidifier. Watch the brightness of the lamp. If the lamp goes dim and stays dim until the breaker trips, you have a voltage drop indicating a wiring problem.


If you have a voltmeter, you can use that in place of the lamp. You can also try using a vacuum cleaner in place of the dehumidifier for step 4.