Black borders / resolution problem on Windows 10

Hi there !

I'm trying to help a family member with her PC, but I'm starting to get out of things to try. Basically, all of a sudden the computer has started to present 2 quite big black borders on each side of the monitor (see here), without any particular incident prior to that.

I'm pretty far away from the PC itself, but I got a couple of remote sessions to troobleshoot things I could think of. Here's what I tried so far:

• Checking resolution from display menu: it shows the correct 1920*1080 one.
• Set the global Windows theme to default: no change.
• Change the frequency from 59,9Htz to 30, then to 60, then back to 59,9: no change.
• Swapping the HDMI cable: no change.
• Lurking through the monitor inboard menu: nothing seems wrong. I didn't find a "factory settings reset" option though.
• No chance to swap another port on the monitor because there's only a single HDMI and D-Sub one... And no possibility to swap the display port on the other end because it's an iGPU with only one HDMI too.
• Run the whole computer through a full MalwareBytes and CCleaner scan: nothing popped.

I was thinking I could try to use DDU, but there's no discrete GPU (3400G on a B450M PRO-M2 max) so there's no point, is it ? I know it's usually pretty "last resort", so I don't want to unnecessarily mess everything up, and even less since I'm not actually in front of the computer myself.