This problem message frequently appears when I dismiss the window or lock my computer while the modpack is running. As soon as I unminimize the window or switch back to Minecraft, it starts spamming the console and logging continuously. Even though it is highly unpleasant, there is no harm (yet). My graphic drivers are all current.
A very old error, used to appear often when mods were used. I know many ways to fix it, but better than me it is described in this article. Read it, try a few ways, should help, good luck.
