You got me.
What version of glibc does your MX-21 use? There have been reports of issues with QtWebEngine and glibc 2.34.
See
https://bugs.launchpad.net/ubuntu/+s...c/+bug/1939993
Also the issue might be old or out of date graphics drivers on the linux side. You might want to try upgrading to a newer graphics driver.
Also, since this is Sigil-1.9.2, you could try disabling the gpu for QtWebEngine (its a brand new Sigil Preference) and then restarting Sigil. Does that make the errors go away?
Outside of that I got nothing ...