A very small and simple installer-project that can be used to reproduce my current problem can be found in the attachments.
This project contains an installer with 2 components (each containing one text file), no component-scripts and an empty control-script for the installer itself.
The work flow to reproduce the issue:
Using the attached project, create an installer.exe, install it, increase the versions of the components, create a repository & upload it, then start the maintenance-tool to update the installation.
Then the maintenance-tool starts updating, asks for admin-rights (which I grant), then I get a "permission denied"-error (see screenshot "install_error.PNG") attached, sorry for the localized error message).
Afterwards, the installation will be broken (see screenshot "corrupt.PNG").