I updated Sweet Home 3D 2.5 to Beta 4 to avoid problems with temporary working files that were wrongly deleted (this happens rarely).
Now, these files are stored in the work directory of your personal application directory (eTeks\Sweet Home 3D\work under Windows, eTeks/Sweet Home 3D/work under Mac OS X and .eteks/sweethome3d/work under Linux).
Please can you confirm it works correctly on your system and that temporary files are deleted once you exited the program?