This is not a bug report because it relates to something that I am sure is considered unsupported and messing around with stuff that you're not supposed to (i.e. QEffects). However, I'm posting it here because I anticipate that, perhaps, some users may come across this "problem", possibly have even forgotten that they have QEffects installed and blame GZDoom when they get undesirable behaviour.
The most obvious problem is a crash on startup.
A slightly odder situation is that, with my own modified build, I do not get a crash. I have no idea why - my modifications are minimal and nothing to do with rendering - perhaps some other option is allowing the game to start, but (if so) I can't figure out which one. However, if the "Camera Textures Offscreen" option is not enabled, I get a smaller copy of the screen overlaid onto the normal view for every camera texture that is in view:
[spoiler]
[/spoiler]Basically, I'm saying it's probably best to avoid QEffects, at least with the latest GZDoom versions that use GLEW. If you do experience problems, check in your GZDoom directory and if you have a file called opengl32.dll in there (plus, probably, a file called QeffectsGL.ini), take the file out of the GZDoom directory and retry GZDoom to see if your problems go away.