"Failed to save file" - no way to save my model

My model does not want to save. When saving it says every time: “Failed to save file”. The same hapens if I try to download the model to my computer to the .skp file. My last working .skp file is 256MB big. I tried to purge the components and materials, but it did not help.

I am a high school student and model a house for a students’ competition.

Is the model too big? Or what can I do to contnue my work and save it?