Something I just found out and might be important for others who use #GameMakerStudio2
There is a relatively new function of the IDE, that is enabled by default.
It removes unused assets, which is definitely good.
But it can also create weird errors, when you have assets (e.g. sprites) used in a way it doesn't anticipate.
Like I am using a sprite in a timeline only (to set a walking animation) and was wondering why the sprite couldn't be referenced.