If this issue only occured on movie DVDs, the Macrovision copy-protection would likely have been the issue, however in this case it may be due to something up with the TV or DVD Player’s video output method.
The first thing I would recommend trying is making sure the Scart output to the TV is set to ‘RGB’. This sends the three individual colour channels directly to the TV instead of via composite, thus leading to a picture with better colour rendering and contrast. Note that some budget Scart leads do not physically have the RGB pins wired up, which means that a composite picture may still appear even if RGB-out is set.
Some TV’s seem to keep the last TV channel ‘tuned’ in the background while viewing the AV and I have often seen faint shadows of the last watched TV channel appearing on AV with some TVs. A simple way to check if this is causing the issue is to turn to an untuned TV channel and then turn to AV. If brightness issue goes away, then this is likely the issue.
Finally if all else fails, I would recommend updating the firmware just in case it is a bug in its existing firmware.