The line is evidently a signal line of some sort - horting it to ground would ensue that it was a signal line of no sort.
"NO STUFF"means "do not install".
It was probably a "design kluge" which was found unnecessary.
It is possible but unlikely that you will find a fault by visual inspection.
The most likely fix that may work is to carefully resolder all joints. Use very fine solder and heat the joint just enough to flow solder onto the joint - or use NO solder and just reflow the joints. This will fix some boards which had a problem initially but soldiered on for days months or years. But, the chances of fixing anything like this without a degree of specialist knowledge is reasonably small.
But, do try - you'll probably learn something.
It MAY be an optical illusion but the IC at the bottom of your photos appears to be badly offset. The red lines at the top are along tracks from the small components (probably resistors).
At bottom either the red lines or the green lines are along the centres of the pins.
IF the red lines are along pin centres then the IC is offset to the right by about 1/2 a pin spacing and would be in immense danger of making wrong or no contact on some pins.
If the pin centres are along the green lines then all is well.
IF the red lines are thepin centres then inspecting the IC pins and ensuring all are properly soldered to where they should be, and tyhat none are soldered to 1 pad across MAY solve your problem.
So, red or green ?