Possible reasons for this are:
a) There is still a process of a web browser active in the background.
> Please ensure to close all web browsers and background processes related to a browser before continuing with the installation.
Tip: With CTRL+ALT+DEL you enter the task manager to look for background processes of the web browser and to close them.
b) A necessary Microsoft Framework is not activated on your PC.
(Microsoft Windows 10 by default uses version 4.5, but the BSSL requires additionally version 3.5)
1. To activate the version 3.5 please navigate to “programs and features” on your PC.
2. On the left side choose "activate/deactivate". (hint: you will need to have administrator permission to do this)
3. A new windows is opening.
4. Click on "+" next to ".NET Framework 3.5".
5. Two additional folders will open.
6. Please activate both check boxes and confirm your entry with "OK".
7. Now Framework Version 3.5 gets downloaded automatically.
8. Once the download is completed, please re-start installation of the BSSL.