Most (Vasili's) plug-ins are sourced from rtl-sdr.ru, this website is currently down and is why the installer is failing.
As morfis said, only use the standard version of SDR#.
The number one problem SDR# will encounter will be from plug-ins.
The community plug-in package is a bit of a dis-service to SDR# as new users can't help themselves and install every plug-in.
They are not made aware that some of the plug-ins are quite old (and no longer maintained) and have become problematic with newer versions of SDR#. There is also the problem that some plug-ins may not be compatible with other plug-ins.
Manually installing plug-ins is not that hard.
Copy file/s to SDR# folder then edit 'Plugins.xml' and add the appropriate line for each plug-in. Done.