Black screen after some unity version changes

So... i've had to reinstall unity , and installed the newest one. Then i came back to 5.4 bcz thats what my project were built in. So i opened up my project...and when i start the application , i get a black screen instead of the usual orange one . same goes when i create the .apk . BUT if i get into another project ...its working well...so whats the problem? what happened? any ideeas? 

Edit: i went in the camera and started it , and went to the ArCamera>camera>backgroundPlane , and it seems that when i start the camera, the mesh renderer turns off...any ideea whY?