Thank you very much for that suggestion. It looks like resolving the problem truly was as easy as uninstalling Qt completely and then reinstalling it again. During the installation process, I selected the default option instead of custom installation and everything went fine. Additionally, the problem with Kits was also resolved.
After installing notepad++ and assigning ".vbs" extension to it, windows wasn't able to execute ".vbs" scripts anymore. Resetting the assignment in the regedit (HKEY_CLASSES_ROOT.vbs to "VBSFile") solves this issue.
Thanks for the input. On WIndows, when you run the Maintenance Tool you check off which compiler you want to use. The Maintenance Tool downloads that compiler. At the point this error happens I have seen the Maintenance Tool download the correct compiler to the Qt directory.
I think you will have to build it from source in order to disable that option
Go to the function void PackageManagerCore::writeMaintenanceTool() and and the function void PackageManagerCorePrivate::writeMaintenanceToolBinary() and just add a return in the beginning of the function to disable it and the maintenance tool won't be included.
Worked for me, but it didn't reduce the size of the resulting installer as I was expecting though.