A google search of the error message produced this:
https://bugreports.qt.io/browse/QTBUG-52397
https://bugreports.qt.io/browse/QTBUG-58322
https://www.mobileread.com/forums/sh....php?p=3951007
So it appears to be OpenGL related and Qt Related. Did you have a large number of Sigil instances open at the same time? Have you tried updating your video driver or make video driver related changes?