This error is 99% due to incorrect directory rights, which means that the TSI cannot import or write the respective files and / or cannot overwrite existing files.
Solution under Linux:
- Log in to the command line of your server / web space and change to the main directory of the TSI
- Execute the following command in the main directory of the TSI from "sudo chmod -R 0777 ./"
- Then run the command "sudo find -name *.htaccess -exec chmod 0644 {} \;" to protect the .htaccess files again
- The bug should now be fixed