It does.
1. If changing to a different shortcut still works to change keyboard layout without any crashing of Sigil, the actual changing of the keyboard layout itself can not be the underlying cause of the crash. Keyboard layouts can be changed successfully even by shortcut with no crashes.
2. Assuming Ctrl+2 works properly on Sigil to make text h2 (when *not* assigned to change keyboard layouts), the actual shortcut is not the problem either.
Given 1 and 2 that implies that Ctrl+2 is probably changing something else on your system besides just the keyboard layout (perhaps changing tabs, changing os workspaces, changing monitors, or ...) which is what is giving Sigil fits and causing the crash.
What bothers me is that if no keyboard layout (say the 4th) exists then Ctrl+4 does nothing (no crash at all) so that is inconsistent in that it implies some relation with changing keyboard layouts and Sigil which is inconsistent with the above.
Is there any error message at all?
Are you 100% sure it is a crash and not just hiding the Sigil window someplace off screen on another workspace or monitor or quitting out of Sigil? Have you tried checking the task manager to verify the program has exited completely?
Also just to rule out QtWebEngine, can you please try reproducing the crash when Preview has been turned off (use the Sigil View menu to hide it)? Does that change anything?
Addendum:
A search of the web showed some hits for similar issues on Windows 10 with other software including Excel, Office 365, and etc.
See for example:
https://adobe-video.uservoice.com/fo...s-aftereffects