Thanks for the information.
Indeed, replacing the 3 OpenTK dll files found in the installation directory of Repetier-Host fixed the issue.
As a side note, the repetier-host installer ignores the installation path typed in and always installs in a directory called Repetier-Host, possibly overwriting another version already there. Not a big deal, but I thought I'd mention it.
The new OpenTK 1.1.1664.6217 shipped with V 1.5.0 works for me.
The old OpenTK 1.1.804.3121 shipped with V 1.5.3 is causing color depth issues.
The problem is not related to the lights in preferences, but it is a color bit depth issue.
With the old version, I need to switch my color depth from 32-bit to 16-bit, and the stl part is rendered in a reddish color, but at least I can see it. If I leave the color depth at 32-bit with the old OpenTK version, it is just a blob without any details. And rendering is very slow too.
With the new version of OpenTK, I can leave the color depth at 32-bit and everything works fine.
Note that for anything to work at all with either the old or new version of OpenTK and avoid an out of video memory error, I need to use slow rendering in Repetier and I need to change the settings in my Intel graphic card settings:
In Graphics Properties, 3D Settings, I change Depth Buffer Bit Depth from Default to 24 (not 16). I also change Driver Memory Footprint from Normal to High (not sure if it makes a difference).
I suspect that the old OpenTK library is not working well with older Intel Graphics cards.
I do not know what issues were caused by the new version of OpenTK so that you reverted to an older version. In any case, I'll keep patching in the new version.
Thanks!