It appears that newer and newer versions of Qt/Sigil (and Visual Studio) may require you to update you Microsoft Visual C++ 2015-2022 Redistributable (x64) runtime.
I just had a failure to launch on an older laptop with a beta version of Sigil using Qt6.9.2. No error, just no launch. Updating the redistributable fixed the problem. Damn Microsoft! These things are supposed to be forward/backward compatible (and they have been for quite a long time), but it looks like something is starting to fall through the cracks. Sigil requires the x64 version of the runtime (in case you don't use both x86 and x64).
Version 14.40.33816 of the redistributable was not good enough on my machine. The latest version of 14.44 restored all functionality.
So head this issue off at the pass and update those runtimes! At the very least, keep this post in mind if Sigil stops launching for you. And spread the word so we can hopefully keep ahead of this!
https://learn.microsoft.com/en-us/cp...?view=msvc-170
POST DATE: 16 September 2025