Originally Posted by KiwiJochen

I see signs of water staining. Hard to see because the photos are all out of focus.

Use the isopropyl alcohol and a toothbrush
Scrub the affected area of the board gently to remove all contaminants
When doing so, hold the board up on an angle so the alcohol and contaminants flow off the board onto eg a towel.

Do both sides of the board

You may need multiple applications to make it clean

Let it dry throughly before reassembly and test

Note that any dull solder should be resoldered. As the board is all surface mount, the right soldering iron, skills and solder are required. Clean again after any resoldering

Good luck.