I had this error yesterday. Luckily I knew which image was causing the problem as it happened just after I added a new enemy. I removed the maps from the project files that I knew had the enemy. Then reloaded the project without the maps and it worked! Then added the maps back to the project folder.
yes this method is best for debugging screw ups. i do it the same way if i have these errors.
even if you don't know which image is wrong, you can trial and error it by backing up the relevant dat file (and maps.dat) and deleting the stuff in the dat. so you can try getting rid of the stuff in maps.dat, then get rid of stuff in textures.dat. if the project opens, you know it is a texture that is named wrong and can compare the backed up textures.dat to the actual images in the textures folder. if it still doesnt open, then it is a misnamed image in decorations or enemies.