I also have encountered this issue after updating the firmware from 6 months old software (cant remember the exact version number if im honest) to the now current 4.0 beta build.
The GUI is bright/vibrant/right contrast no problems with the calibration of the TV right up to either viewing a jpg of the file (doesnt matter what file it is movie,tv series, jpg, however I cant replicate it, its seems to be random) were the contrast and brightness drops so dramatically that its near impossible to view anything while its being played. Once the file is exited, the brightness/contrast returns back to its original state and I can navigate the browser/explorer with ease as normal.
The only way that Ive found to cure this is to reflash the media centre with another version of software to completely erase and replace it.
This has now happened twice since friday evening since updating it and obviously hasnt happened before this with previous versions.. Im considering rolling back to the last version of V3 to cure this issue as its rather frustrating not being able to view anything after it happens
Now that I know someone else are having the same issues, its not me pressing buttons by accident that is causing it.
My player is the 800X3D
Edit: To support this issue.. Ive created a small video taken on my phone. As you can see the brightness is all ok, I press play and the video is instantly dark. (You maybe able to make out the resume play window in the centre) I stop the video and the brightness is back fine again. Please also notice that the background darkness/brightness doesnt not change so this is not the phones auto balance adjusting. Im pretty well up with tech btw
The video is 24mb and is available here:
http://vidmg.photobucket.com/albums/v112/markyb555/VID_20150223_2037301.mp4Thanks