Your web-browser has many settings/options.
For each file-type download made through your web-browser you can/must set an action...
So for example, for a PDF file you might specify to always open it with the associated PDF-app...
For a SKP file you might specify to either open if [with SketchUp using a Temp-file download] or to Save it directly to your computer - with no 'open'...
For an RBZ [which is an extension archive] you should always have it set to Save... and nothing more...
For the Saving location you can specify [say] to use your 'Downloads' folder, or 'Ask for location' each time...
It sounds like your browser-settings have RBZ associated with SketchUp [incorrectly] and so it tries to open it after it is downloaded, and thereby breaks SketchUp doing.
You need to Save it, and then use the built-in SketchUp > Extension-Manager to 'Install' the RBZ for you...
So change your web-browser's settings/options to save the RBZ, NOT to open it...
Then retry getting an RBZ downloaded... and then installed...