Spray the header pins with DeOxit contact cleaner (spray it on and then install and remove the connector a few times) and that should do it. Try the DeOxit on the fuse holders, but the brackets may need additional abrasive cleaning.

A bigger issue is you've got some cold solder joints on the back of the board. In the third photo there are four joints on the bottom left. See the circular crack around the joint, that's a problem. They may be ok for now, but they won't last much longer. Check the rest of the board for the same issue.

Clean the joints with a fiberglass brush, add a little flux and then reflow the connections by adding a small amount of solder.