Line fault at the outlet -- do I need an electrician?


Yesterday, I got a Panamax, Max 1500 surge protector and line conditioner. (I got a very good deal on it, and am just trying it out.)

I plugged it into an outlet I've been using for a while and one of the red lights on the front lit up saying "line fault." (I'm not sure how this is different from a "ground fault." Maybe it's the same.) The Panamax does not do this with other outlets in the room. They seem ok.

So, I know this means that the outlet is improperly wired. My question is, might this be a simple thing to check and/or fix? Any suggestions most appreciated. It's the only outlet I can use to have my audio set up where I usually have it. Now is not an optimal time to call an electrician. If this is a big problem, I'll try out my gear somewhere else in the room, but if I can fix this without too much expertise, that would be ideal.
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An image of the multimeter is here:

www dot jbryant dot eu/pages/DMM dot htm

I used the scale in the upper right that says V~ and has the numbers 600 and 200.
I set it on the 200 scale.
I re-tested and got a 3.7 reading.

I still need to unplug everything and get back to you.
OP again:
I'm pretty sure I unplugged everything on the chain of outlets.
I have not thrown the breaker yet.
All outlets on chain read as reversed:
*  the negative plug input reads as 120v, more or less
* the positive plug input reads as 3.4v, more or less
* Unplugging all things in the outlets made no difference to the 3.4v reading.
* On other chains in the house, things are not reversed. They read zero from the negative input plug.

Your wires are reversed in the circuit somewhere you need to take some plates off and start looking at the wires.

Reversed hot and neutral wires. Measuring hot-to-neutral voltage by itself doesn’t tell you if those wires are reversed. You must measure neutral-to-ground or hot-to-ground voltage. If the neutral-to-ground is 120V and the hot-to-ground is a few volts or less, then the hot and neutral wires are reversed


https://www.ecmweb.com/content/article/20900908/diagnosing-power-problems-at-the-receptacle
I'm somewhat perplexed by the latest measurements, and I can't readily envision a miswire (or even multiple miswires, perhaps at the panel as well as at one or more of the outlets) that could account for these findings.  But what I would say at this point, given the possibility of multiple unknown miswires, is DO NOT ASSUME THAT TURNING OFF THE BREAKER WILL KILL THE ELECTRICITY TO THE OUTLET.  

Best,
-- Al