I would recommend you to 'disassemble' your level (don't forget to make a backup). Use 'default' texture instead of all textures that your second room has (if your levels are small, this won't be a problem I guess), use 'CUBE_MDL' for models that you create on fly via script, and use simple cube model (just create it in MED, simple cube) instead of all models that your second level has. If it won't crash, then start adding each model one by one. Still no crash? Same for textures. This will help to find out, if it's related to script of models/textures. If at the end, you will end up with getting your level back as it was originaly, then it's 100 % related to code. I'm pretty sure that there won't be easy way out of this.

Greets