U700 Strange Issue After Switching On The Camera

[producer]

Hi,
After I replaced the flex cable, my U700 works as new.
But: when I switch the camera on, the sensor buttons start continuously blinking very fast (sometimes only the "call" sensor button) or even they all become inactive with no light. The camera and the whole software become stuttering and very slow even after sliding the upper panel down and after few seconds the phone returns to its normal working condition.
Still can take a picture with camera side-button and once the picture is taken and the display shows the picture preview, everything goes back to normal - all the sensors become active again. Until the next camera mode...
So, everything works perfectly until I switch the camera on.
I was looking everywhere inside, even put some isolation bands between the plates - the same problem.
Any help and suggestions, please?
Anyone experienced the same problem?
Big thanks in advance!