This resulted from CMake issue #22265.

When building qtbase with MSVC and -debug-and-release -force-debug-info in a build dir of sufficient length, e.g. z:\build\6.1.1-windows-x86_64-msvc2019\qtbase, then the build fails with

...
z:\build\6.1.1-windows-x86_64-msvc2019\qtbase\src\plugins\networkinformationbackends\networklistmanager\QNetworkListManagerNetworkInformationBackend_autogen\mocs_compilation_RelWithDebInfo.cpp : fatal error C1083: Cannot open compiler generated file: '': Invalid argument

See further discussion in CMake issue.

This can be easily reproduced with the following configure flags

../qtbase/configure -debug-and-release -no-gui

in the build directory

z:\build\6.1.1-windows-x86_64-msvc2019\qtbase1234567

With the original build dir z:\build\6.1.1-windows-x86_64-msvc2019\qtbase one needs -force-debug-info.
That increases the configuration name length by 7 (Release vs RelWithDebInfo).