Viewer will not launch,

[redbeard5598]

Calibre's epub viewer refuses to run, and always gives the same error. From the command line on Linux:
ebook-viewer
QGLXContext: Failed to create dummy context
WebEngineContext used before QtWebEngine::initialize() or OpenGL context creation failed.
Failed to create OpenGL context for format QSurfaceFormat(version 2.0, options QFlags<QSurfaceFormat::FormatOption>(), depthBufferSize 24, redBufferSize -1, greenBufferSize -1, blueBufferSize -1, alphaBufferSize -1, stencilBufferSize 8, samples 0, swapBehavior QSurfaceFormat:efaultSwapBehavior, swapInterval 1, colorSpace QSurfaceFormat:efaultColorSpace, profile QSurfaceFormat::NoProfile)
Aborted (core dumped)

If I can get the GUI to run, it gives the same output, without the core dump or the Abort. Not an expert, so suggestions? It worked fine until about a week ago. Ubuntu 18.04. Looks like Calibre 4.99.12. Now, I'm hoping that updating will take care of the problem, doing that as I type, and I'm guessing that something updated that it was depending on. Except that the update didn't fix the issue; I still get the errors in 5.10.1. After clicking "View" from the Calibre GUI, I see this in terminal:
WebEngineContext used before QtWebEngine::initialize() or OpenGL context creation failed.
QGLXContext: Failed to create dummy context
Failed to create OpenGL context for format QSurfaceFormat(version 2.0, options QFlags<QSurfaceFormat::FormatOption>(), depthBufferSize 24, redBufferSize -1, greenBufferSize -1, blueBufferSize -1, alphaBufferSize -1, stencilBufferSize 8, samples 0, swapBehavior QSurfaceFormat:efaultSwapBehavior, swapInterval 1, colorSpace QSurfaceFormat:efaultColorSpace, profile QSurfaceFormat::NoProfile)
How do I fix this? Not even sure what the issue is.

[kovidgoyal]

It means your GPU driver is incompatible with Qt WebEngine with the viewer uses. You can try turning off hardware acceleration, as described here: https://doc.qt.io/qt-5/qtwebengine-debugging.html

[redbeard5598]

Originally Posted by kovidgoyal

It means your GPU driver is incompatible with Qt WebEngine with the viewer uses. You can try turning off hardware acceleration, as described here: https://doc.qt.io/qt-5/qtwebengine-debugging.html

Nope. doesn't work.
ebook-viewer: error: no such option: --disable-gpu

[PeterT]

Originally Posted by redbeard5598

Nope. doesn't work.

ebook-viewer: error: no such option: --disable-gpu

I think you need to set the QTWEBENGINE_CHROMIUM_FLAGS environment variable (see that same web page)