Hi Scott,
sorry for the inconvenience with the installer.
Errors related to "qmake.exe -query is not parseable" happens usually because:
<your_qtsdk_root_dir>/5.12.3/msvc2015_64/bin/qmake.exe does not exist
<your_qtsdk_root_dir>/5.12.3/msvc2015_64/bin/qmake.exe can not be executed
For the (1) there can be multiple reasons:
Releasing/Packaging bug (dependencies in component .xml files -> the component is not extracted on disk prior to trying to access files (qmake.exe) from it)
Installer-Framework bug, it failed to extract the archive (containing the qmake.exe) to disk
insufficient disk space (installer should warn about this though)
For (2) the reasons are usually:
directory permission issue, you can try installing with admin rights but generally this is not recommended, try to install to directory where the user has permissions
virus scanner kicks in
trying to install & run 64bit Qt executables on 32bit machine
Could you try to rule out the obvious ones first? If the issue persists please open a bug report with as much details as possible.
You can also check the following:
run the installer as before
once the error is shown by the installer do not continue/abort, just leave the session there
open another terminal/file browser
go to the "<your_qtsdk_root_dir>/5.12.3/msvc2015_64" directory, what files are there? If qmake.exe is there can you invoke the "qmake.exe -query"? How are the permissions for the directory and files?