Godot Engine v3.1.2.stable.official - https://godotengine.org
OpenGL ES 2.0 Renderer: GeForce GTX 1050/PCIe/SSE2
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ERROR: Node not found: handcuffed
   At: scene/main/node.cpp:1382
SCRIPT ERROR: rebuildparticipantslist: Invalid set index 'visible' (on base: 'null instance') with value of type 'bool'.
          At: res://files/scripts/newsexsystem.gd:636
ERROR: Node not found: handcuffed
   At: scene/main/node.cpp:1382
SCRIPT ERROR: rebuildparticipantslist: Invalid set index 'visible' (on base: 'null instance') with value of type 'bool'.
          At: res://files/scripts/newsexsystem.gd:636
ERROR: Node not found: handcuffed
   At: scene/main/node.cpp:1382
SCRIPT ERROR: rebuildparticipantslist: Invalid set index 'visible' (on base: 'null instance') with value of type 'bool'.
          At: res://files/scripts/newsexsystem.gd:636
ERROR: Node not found: handcuffed
   At: scene/main/node.cpp:1382
SCRIPT ERROR: rebuildparticipantslist: Invalid set index 'visible' (on base: 'null instance') with value of type 'bool'.
          At: res://files/scripts/newsexsystem.gd:636

Yes, it's a known issue. The problem can be avoided by simply un-equipping the handcuffs during the sex interaction.

To fix the error yourself before the next version comes out you will need to edit "/scripts/newsexsystem.gd" with a decent text editor and make the following changes. (Leading spaces will need to be changed to tabs; 4 spaces to 1 tab)

replace line 2186 with:
        elif member.isHandCuffed:
replace line 152 with:
                    isHandCuffed = true    
        
replace line 88 with:
    var isHandCuffed = false