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

If you’re ok with pulling the outlet cover off and shining a flashlight in on both sides you may be able to see the hot (black) and neutral (white) wire going to the side screws, or if it is a back-stab connection you may be able to see which side black goes to and which the white goes to.  These wires need to be on the correct color screws on either side. ( Silver screw and brass screw. )

see attach link for details if you live in the US.  
https://i.pinimg.com/originals/a4/ba/86/a4ba86ca2eaa2d769727076a912e4452.jpg
If the colors are reversed from the image you have reverse polarity at the socket.  If correctly wired, as mentioned above you may have a loose screw connection or defective back-stab connection.  All are relatively simple corrections if you’re comfortable turning off breaker and redoing the wire connection.  Electrician is the certified answer but a quality handyman is quite capable of the same job at a lesser cost if you’re not up for the task.  

Thanks, Bryhifi. I think I could at least take a look. It's two different outlets that have the fault -- one feeds the next one. I wonder if correcting the root might also correct the branch.

hilde45,  that is a possibility if the feed going to the line side of the first outlet is reversed and the daisy chained outlet is wired in parallel off the load side of the first outlet in correct polarity. 

https://solitum.net/content/images/2015/08/62304C67F7C8465FB3B153929E16E752.jpg

Yes, it's usually the first one that needs fixing which fixes the next in line.
I would get that plugin tester for the rest if the house anyway and make your job easier.
Good luck

Indeed a quick answer to the question of polarity is the three prong outlet tester.  https://target.scene7.com/is/image/Target/GUEST_f8afe67f-c81b-4adb-bf20-4492211bf748?wid=488&hei...