Outlet works with fans, but not with portable air conditioner

I'm not an electrician, and not particularly electrically savvy. What I describe below is what I've learned (I think) over the past few days from reading online.

I bought a RolliBot RolliCool COOL 100H-20 portable air conditioner a few week ago. I installed it and powered it up on June 15. Everything worked as expected.

At the end of my day yesterday, I powered it down and went to bed.

This morning when I woke up, I turned the 100H-20 on, but there seemed to be no power to the device.

I unplugged the 100H-20 and plugged in window fans to check the outlet (top and bottom). The window fans - one two-prong and one three-prong - turned on. I unplugged the window fan and plugged the 100H-20 back in to that same outlet, but there was still no power to the 100H-20.

There appears to be a circuit breaker on the 100H-20 plug, but it had not been tripped. Still, I reset it.

I tried turning the unit on through the wi-fi app, but the 100H-20 was not connected to the network (understandably).

I checked my home circuit breakers, but none had been tripped (and all other devices on that circuit were working).

I unplugged the 100H-20 and plugged it into a different nearby outlet - on the same circuit - and the 100H-20 immediately powered up.

I tested the "bad" outlet with a "E150282 SS-500 110-125V 0.3W CIRCUIT TESTER" and both top and bottom showed "CORRECT".

The circuit is comprised of six outlets which power several computers, monitors, and the entertainment system. It has a 20 amp breaker. I know I'm pushing it. The breaker tripped once when I had too many devices turned on, but the 11.5 amp 100H-20 draws the most power. Since that breaker tripped, I've been careful to turn off all devices that I'm not actually using.

I'm guessing the other devices draw only about 5 to 12 amps (in a typical conservative use situation). The spec plate on the 100H-20 reads 11.5 amps. That totals 16.4 to 23.7 - enough to trip the breaker, if I'm not careful.

The condo I live in was built in 1984, so I'm a little concerned about old wiring and fire hazard. But before I spend money I don't have on an electrician, I'm wondering if maybe I'm the only one confused about this, or if someone else may have some ideas I haven't yet thought of.

Thanks much!