I deduce USA NEC as your code, from the coloring of the wires in the picture. Below explains the problem and how to fix it.

NEC requires the going and returning current to be in the same wire bundle (cable or conduit), so the magnetic fields thrown off by the current in the wires cancel each other, thus preventing magnetically inducing current flow in adjacent metal objects, which could cause heating in the adjacent objects and possibly fires.

In the **left side** of your picture, you appear to have one end of a standard 3-way light switch arrangement:

[![good 3-way with balanced wires][1]][1]

Power comes in on one or the other of the T travelers, goes through the 3-way switch to the common screw C then to the black L to the light, then returns on the white L from the light to the white in the traveler bundle. Magnetic fields are balanced in the LL cable and in the TNT cable.
**The left side is good**, you don't need to adjust it, except to take out the right side's neutral.

*********************

The **right side** is problematic. It looks to me like someone wanted a light that only had one switch controlling it to have two switches in a 3-way arrangement, and wired the light illegally to achieve this goal.

What probably used to be there was this:

[![normal one-way switch][2]][2]

Power came in on black T, through the switch to black L to the light, then returned on white L to white T. Magnetics were balanced in both the TT and LL cables.

Here's what you've got now:
[![bad 3-way with no neutral coming in][3]][3]

To get the one-way-switched light to be a three-way-switched light, they illegally re-purposed the black and white TT cable to be travelers. But there's no wire for the return current in the TT cable. So they bootlegged off the other neutral in the box.

So power is now coming in on the black and white TT "travelers", through the 3-way switch to the black light L, returning on the white light L, then going off to another circuit's neutral. While the LL cable here is balanced, the TT "travelers" don't have a balancing neutral in them, so they're throwing off magnetic heating inside the wall. The return neutral power is going on another circuit's neutral, so that current is extra in the other circuit,  unbalancing its magnetics, also heating the wall.

Note that even if this is a multi-wire branch circuit, the neutral return current is not balancing the power current within the same cable. This is a problem. 

**To fix it**, you'll have to convert the right side back to a one-way.

Turn off the power to both circuits and check for voltage. Then remove the right side white L from the left side bundle. Remove the white T from the switch traveler screw. (You can keep the existing 3-way switch in place: a 3-way can be used in a normal one-switch circuit simply by not using one of the traveler screws.) Connect the white T and the white L together under a tight wire nut or wago. Your goal is to get back to the "What probably used to be there" picture above.

Then you'll have to fix the other end of this bad 3-way circuit. Open the other end box, and you might find something like this:

[![bad 3-way other end][4]][4]

Power and neutral come in on the PP cable. White P dangles, connected to nothing. The travelers TT take the power onward, but the neutral return current doesn't come through these cables, so the magnetics heat the wall.

Remove the switch: Connect black P and black T under a wire nut or wago. Connect white P and white T under another wire nut or wago. 

Now the light is back to a one-switch and legal wiring.

***"But it was convenient to have the light 3-way!"***

Devices are available that can convert a one-way light circuit to a 3-way circuit safely and legally, that don't require changing the cables in the wall. Swap one of these devices in for the switch, and possibly for the removed switch, depending on the device instructions, and you'd be back to 3-way safely and legally.

If you don't want to rely on a special device, you'll have to replace the cable now being used as illegal TT travelers with a three-wire cable like the left side has. 


  [1]: https://i.sstatic.net/8MH51YhT.png
  [2]: https://i.sstatic.net/FW9PrQVo.png
  [3]: https://i.sstatic.net/okOwmzA4.png
  [4]: https://i.sstatic.net/XrA8Opcg.png