Hi @anon29869905 , thank you very much for your help!
As the concerned system is running on a Win7 VM I checked the logs in [install dir]\otrs\var\log\otrs
Unfortunately there is no new error log entry, when attempting to install the migration plugin.
To test the functionality I tried to reinstall the Znuny repository plugin, which gave a proper error both, in the browser and inside the error log:
[Mon Mar 2 11:04:25 2020][Error][Kernel::System::Package::PackageUpgrade][677] Can't upgrade, package 'Znuny4OTRS-Repo-3.3.2' already installed!
But as stated before, there is no new log entry, when trying to install the migration plugin.
Am I missing something out here? Maybe it’s still an issue with the IIS7 config?