Dunno exactly how much info you need, so here's exactly what I did a few minutes before the error started appearing.
I had just started on the winter level ("Level_4" layout name). I had copied over some essential objects from Level_1. I then added a level image from Tiled. For some odd reason its size made C2 distort the image in the preview, so I deleted it, resized the image, then added it again. I then went to add the music (Level_1_Intro , Level_1_Main) but discovered I already did that, so I deleted the duplicates. I then added the events to play & loop the music at the start of the layout, then went to preview.
It was at this time I got an error and the preview wouldn't load. I think I just lost my internet connection so I shrugged it off, saved my project, and closed C2. When I went to open it later I got the error I posted above and now the game won't load.
Sending you the .capx now. Thanks for looking into it!