Game version : 0.5.25
Your OS or if you are playing web version: WIndows 10 (there's a web version?)

Bug description: 
I'm not sure what it does but I got error message when initiating sexscene.

SCRIPT ERROR: startscene: Invalid get index '0' (on base: 'Array').
At: res://files/scripts/newsexsystem.gd:1196

Note: I'm using old save game from 24, so maybe it's not a bug afterall.

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Quick suggestion: Maybe you can add  a few rare slaves. Like something you can collect? Maybe similar to Tia (?) from Cali's side quest. With two or three positive traits, but more difficult to tame and higher chance to escape even while wearing handcuff..........
Okay, now I saw it in writing, I guess it will be more complex than I thought. (I haven't got far progressing in story, so if something like this is already exist later in the game, just scratch it)

The error means that somehow you had an ongoing fucking action with either zero givers or zero takers.  Unlikely to be the result of using a save from 0.5.24.  More likely that either a check was missed or a bad assumption was made somewhere in the code.  Without knowing a specific set of actions during the interaction that triggers it, there is little that can be done.

Slaves all behave roughly similar once captured, though some NPCs have quests that continue in your mansion.  The acquirable NPCs are pretty much the rare slaves of the game, as they have unique traits and pictures.