Oh, I know, I've reported quite a few bugs over the years. What I was saying is that I didn't find any reference to this issue on Jira.
By the way, I have a workaround for the issue. I've attempted debugging Qt sources on the machine where loading qwindows.dll fails, and noticed that QCoreApplication::libraryPaths returns empty list, while it does return a valid path on the machine where it works fine. So, I added this one line to the main function before QApplication is created:
if (argc >= 1 && argv != nullptr)
QCoreApplication::addLibraryPath(QFileInfo(argv[0]).absolutePath());
And now my application runs again, on all the PCs where I've tested it!