When you run into this error there are a few things we would start with to troubleshoot.
Sometimes when you have a lot of objects, tiles, or a very large custom map you might run into this issue. This happens to avoid adding too many things to the space that are required to load individually, which can put excess strain on participants CPU and result in a poor experience for that participant.
If you run into this error, we recommend creating separate rooms with smaller footprints to divide some of the objects into different rooms. This will help disperse some of the objects that need to be loaded and allow participants to load them properly.
This will require more design work, but you can use a third party software like Tiled to create custom backgrounds with objects built into the background. This will allow them to load as a part of the background image, instead of individual objects within the space.